‘Hey Presto! Better Queso Fresco, with High Pressure Processing

That’s according to Tomasula et al. writing in the January issue of the Journal of Dairy Science​, who nonetheless warn that there is a clear cost implication.

Queso Fresco is a Hispanic soft cheese distinguished by its bright white texture, crumbliness, mild salty flavour and non-melting characteristics.

But Tomasula et al. warn that its high pH and moisture content provide the ideal conditions for bacteria and other microflora to growth, which can limit its shelf life.

“Despite the use of pasteurized milk, Queso Fresco and other fresh cheeses made commercially have been subject to occasional recalls, most likely due to environmental contamination by Listeria monocytogenes,”​ they write.

Listeria contamination in soft cheese​

They explain that this can occur both on the surface and interior of the cheese due to processing steps that may involve handling of the curd and use of utensils, and during the milling stage that is used to impart a crumbly texture to the curd.

Exploring the potential for HPP as a post-packaging listeria intervention, Tomasula et al. used an AVURE 2L-70 system with a 690MPa (mega-Pascal) pressure limit and a 10-90C operating range.

Using a pressure medium of filtered distilled water, the team employed pressures of 200, 400 and 600 MPa to treat the Queso Fresco, which was inoculated with Listeria – via cheese curds before molding or slicing or via the surface of cheese slices.

The scientists found that using HPP with the cheese at 20C, a maximum pressure of 600 MPa and a hold time of 20 minutes eliminated L.monocytogenes​ in curds or on the surface of the cheese slices.

Pathogen growth resumed after seven days when the listeria was inoculated within curds, and 28 days, when it had been added to the surface of slices.

“High hydrostatic pressure processing in conjunction with anti-microbials may help limit the growth of L. monocytogenes during 4C storage – adjustments in pH, salt content or water activity of Queso Fresco may also help limit growth,”​ Tomasula et al. write.

One alternative was adding salt, but Tomasula et al. say additions in the 5-11% range “would undoubtedly affect the properties and flavour of the cheese”.​

What is the cost implication?​

Summarizing their study, the team said was effective in eliminating the various microorganisms that can limit the shelf life of QF, and calculates the annual cost of installing and running a $2.2m AVURE system at $0.13kg/cheese for three minute process and $0.26 for 10 minutes.

“This suggests that, at the conditions of this study – 600 MPa, initial Queso Fresco temperature of 20C – HPP is a promising post-packaging process that targets spoilage micro-organisms thoroughout the cheese and not just at the surface,”​ Tomasula et al. write.

“This would be an advantage over the use of anti-microbials, which would have to be applied throughout the cheese curd and possibly alter the flavour of the cheese,”​ they add.

Title:​ ‘Effect of High Pressure Processing on Reduction of Listeria Monocytogenes in Packaged Queso Fresco’

Authors: ​Tomasula, P.M., Renye, J.A, Van Hekken, D.L., Tunick, M.H., Kwoczak, R., Toht, M., Leggett, L.N., Luchansky, J.B., Porto-Fett, C.S., Phillips, J.G.

Source: ​Journal of Dairy Science, ​January 21 2014, 97:1281-1295, http://www.journalofdairyscience.org/article/S0022-0302(14)00048-4/abstract​

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