DIFFERENT NONCONVENTIONAL TECHNOLOGIES IN THE DAIRY SECTOR

HIGH-PRESSURE PROCESSING (HPP): GENERAL CONCEPT

High-pressure processing (HPP) is an approach at room temperature for food processing and preservation. This promising method is also known as high hydrostatic pressure (HHP), (Figure 1.1). The products are usually packaged and loaded into a treatment chamber. The treatment chamber is filled with a fluid capable of transmitting pressure-(nonnally water), pressurized with a pump at high pressure (100-1000 MPa). The pressure is transmitted uniformly to the food while the temperature slightly increases (Bunzrul et al., 2008). To prevent heat-induced damage, the temperature in the treatment chamber is controlled. The HHP is capable to inactivate foodbome spoilage and pathogenic microorganisms without affecting the food’s quality which includes nutritional (like vitamins) and sensory (Шее aroma and flavor) (Tao et al., 2014). Hite

(1899) demonstrated for the first time that food shelf-life could be extended by HPP treatment. In 1991 in Japan appeared the first HPP-treated products.

Schematic representation of high-pressure processing of food

FIGURE 1.1 Schematic representation of high-pressure processing of food.

MECHANISMS OF ACTION ON MICROBIAL INACTIVATION

In HPP, microbial inactivation occurs by means of damages of the membrane, denaturation of proteins, and reduction in intracellular pH (Li and Farid, 2016). The microbial resistance to pressure is variable depending on several factors viz. type of microbes, processing conditions (holding tune, temperature, and pressure), food composition, pH, and water activity (aw). On the other hand, the susceptibility to treatment is influenced by the growth stage of microorganisms. In the exponential growth stage, the microbial cells are more sensitive to the effect of pressure. Instead, bacterial spores are more resistant, up to the pressure of 1000 MPa, compared to vegetative cells. However, the combination of HPP and heat is able to inactivate foodbome bacterial spores (Li and Farid, 2016). Compared to gram-positive bacteria the gram-negative bacteria seem to be more sensitive to pressure. In fact, it was observed that gram-positive organisms need 500-600 MPa at 25°C for 10 min to be inactivated, while gram-negative 300-400 MPa under the same conditions (Chawla et al., 2011). This technology can help to reduce thermal degradation respect to conventional processes, being the pressure distribution rapid and uniform throughout the sample. Furthermore, pressure can accelerate the inactivation kinetics of microorganisms. HPP does not affect sensory and nutritional properties (Chawla et al., 2011).

APPLICATION OF HIGH-PRESSURE PROCESSING (HPP) TO DAIRY SECTOR

Various studies have widely evaluated microbial and enzymatic inactivation, denaturation of whey proteins, and changes in the coagulation properties of milk after HPP treatment (Huppertz et al., 2006; Considine et al., 2007). 400-600 MPa of HHP treatment is effective on raw milk as the pasteurization process. Milk treatments with HPP at 400 MPa or 500 MPa for 15 and 3 minutes respectively are sufficient to obtain 10 days of storage at 10°C (Rademacher and Kessler, 1997). Instead, the application of HPP at 600 MPa with a treatment temperature of 65°C for 12.5 min significantly reduced C. perfrigens spores in milk (Gao et al., 2011). In cheese production, the pre-treatment of the milk with HPP increase cheese yield-reducing rennet coagulation time (O’Reilly et al., 2000; Canninati et al., 2004; Rynne et al., 2008; Okpala et al., 2010; Chopde et al., 2014). HPP treatment applied to cheese can have an impact on its physicochemical, microbiological, and sensory characteristics. From the available literature, the use of the specific range of pressure can be allowed a desirable change in a physicochemical, microbiological, and sensory attribute of treated cheese (Rynne et al., 2008; Okpala et al., 2010). The use of HPP at the end of the cheese manufacturing process could reduce the levels of undesirable microorganisms, thus increasing cheese safety and storage. Some studies focused on HPP to reduce the microbial load of E. coli, S. aureus and L. monocytogenes on fresh cheese (Cappellas et al., 1996; O’Reilly et al., 2000; Canninati et al., 2004; Lopez-Pedemonte et al., 2007). Cappellas et al. (1996) evaluated the effect of the different combination of pressure (400, to 500 MPa), temperature (2, 10, and 25 0 C), and treatment time (5 to 15 min) on the lethality of E. coli inoculated in white cheese from goat’s milk. No surviving E. coli were detected at 15, 30, or 60 days in any case, and a positive effect on the shelf-life extension was observed. The microbial inactivation of HPP (pressure 50 to 800 MPa; time 20 min; temperatures 10, 20, and 30°C) on Cheddar cheese was also studied. Pressure treatment at 400 MPa for 20 min at 20°C reduced the numbers of viable S. aureus and E. coli by 3- and 7-log units, respectively (O’Reilly et al., 2000). In other studies, the effect of HPP from a microbiological and sensoiy point of view was evaluated by Caiminati et al. (2004). In their study, L. monocytogenes was inoculated on the rind of Gorgonzola and it was observed that pressures higher than 600 MPa for 10 min or 700 MPa for 5 min significantly reduce the microbial growth. Furthermore, the authors reported that only one of the four pressurized cheese samples was compromised from the sensory point of view. These results were also confirmed by Lopez-Pedemonte et al. (2007) who inoculated L. monocytogenes in a model cheese and analyzed the effects of 10 min HPP (pressure 300 to 500 MPa) at 5 or 20°C, concluding that 400 MPa represented the most appropriate choice to achieve significant reductions of pathogen counts without affecting the starter cells.

 
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