Isothermal inactivation of Mycobacterium avium subsp. paratuberculosis in curd simulating the stretching phase in pasta-filata cheese process

Abstract

Raw milk and dairy products are usually considered the major sources of Mycobacterium avium subsp. paratuberculosis (MAP) exposure for humans. During the production process of mozzarella cheese, as well as of other pasta-filata cheeses made with pasteurized or raw milk, curd is heated and stretched by addition of hot or boiling water. This step is the critical point for the inactivation of MAP during the production process, but, to our knowledge, no studies have been published about the thermal death time values of MAP in curd. The aim of this study was to determine the inactivation kinetics of MAP in curd used to produce pasta-filata cheese in six independent experiments. The milk was inoculated with a mix of MAP strains (field and registered strains) and, with the aim to simulate the thermal treatment of the curd during the stretching step, samples of 10 g of contaminated curd were vacuum packed and treated separately at six different temperatures from 60°C to 75°C in a water bath. MAP survival was then evaluated by plate count method and inactivation parameters were estimated for determining the thermal resistance of the pathogen directly in the curd. D-values increased from 0.15 min (D₇₅-value) to 4.22 min (D₆₀-value) and the calculated z-value was 10.2°C. These data aid: (i) to design food thermal process treatments defining acceptance limits of critical control points to ensure safety against MAP; (ii) to predict the time/temperature combinations needed to obtain a certain MAP log reduction during the curd stretching step; (iii) to optimize or validate pasta-filata cheese process.

Keywords: Mycobacterium spp.; food safety; inactivation models; paratuberculosis; pasta-filata cheese; thermal resistance kinetics.

Figure 1

Pictures collected during the experimental trials and representative of curd maturation under whey (A), mature curd (B) and vacuum-packed curd samples before the thermal treatment (C).

Figure 2

Representative thermal inactivation curve of MAP in mature curd treated at 64°C. Symbols are the plate count of MAP. Line is the curve generated from the “log-linear +Tail” model (Geeraerd et al., 2000).

Figure 3

Log Survival curves determined from the log-linear portion of the inactivation curves (solid lines) for MAP (symbols) in curd during the simulation of the stretching step at different temperatures (isothermal treatments). Each curve represents an independent experiment.

Figure 4

Thermal death time curve (z-value) for MAP in curd treated from 60°C to 75°C obtained by D-values in a log scale (symbols) as function of treatment temperatures.