Nonthermal Pasteurization of Beer

Abstract

The industrial production of bottled beer ends with a process of thermal pasteurization. This heat process aims to inactivate the fermenting yeast and potential spoilage microorganisms, and increase the shelf-life at room temperature. However, the conventional thermal process can affect the original beer flavour and freshness. This research investigated beer pasteurization using nonthermal technologies, namely High Pressure Processing (HPP), power ultrasound, and Pulsed Electric Fields (PEF). The logarithmic reductions in Saccharomyces cerevisiae ascospores were determined for all the technologies and compared. The kinetic parameters of the inactivation of S. cerevisiae ascospores in beer with different alcohol content were estimated for conventional thermal processing, HPP and power ultrasound, and minimum pasteurization conditions (15 pasteurization units, PU) were recommended. In addition, sensory assessment was carried out for thermally, HPP, TS and PEF treated beers. The specific energy requirements for equivalent log reduction of ascospores (15 PU) by HPP, PEF, thermosonication (TS), and thermal processes were compared. Lastly, the morphology of untreated, HPP and thermally treated S. cerevisiae ascospores in ascus and single spore form were studied to explain the underlying mechanisms of spore inactivation. While thermal inactivation of S. cerevisiae ascospores followed the first order kinetics, the inactivation by HPP and TS was non-linear with concave upward. Different models were attempted and results demonstrated that Weibull model was the best fitted for the inactivation of ascospores in beer by both HPP and TS. The first-order thermal resistance parameters of Saccharomyces ascospores in beer were estimated for four yeast strains: D60˚C-values of 11.2, 7.5, 4.6, and 6.0 min and z-values of 11.7, 14.3, 12.4, and 12.7°C were determined for DSMZ 1848, DSMZ 70487, ATCC 9080, and Ethanol Red®, respectively. D55°C-values of 34.2 and 15.3 min were obtained for 0 and 7% beers, respectively. For HPP, the log reductions in ascospores were 3.1, 4.9, and ≥6.0 in 0.0, 4.8, and 7.0% alc/vol beers processed at 400 MPa for 10 min, respectively, and the Weibull model was suitable for explaining the nonlinear inactivation of ascospores by HPP. With respect to sensory assessment, a triangle test revealed no significant difference in the overall flavour of untreated (control) and HPP-treated beers. The effect of ultrasound at ≤23°C on the S. cerevisiae ascospore inactivation in 0.0, 4.8, and 7.0% alc/vol beers showed that power ultrasound alone was not enough for the pasteurization of beer. Continuous TS operation (0.53 mL/s, T≤70°C) was not sufficient to pasteurize the beer (≤1 log reduction), but batch operation was sufficient. Similar to HPP, the Weibull model fitted well the log survivorship curve. TS at 50°C for 3.0, 1.9, and 4.5 min could deliver the minimum pasteurization of beer with 0.0, 4.8, and 7.0% alc/vol content, respectively. It was found that the TS treated beers developed a haze and the preference tests showed less preference for the ultrasonicated beer. The nonthermal PEF (T≤43°C, 45 kV/cm electrical field intensity, 46 pulses, 70 μs) inactivation of ascospores revealed 0.2 and 2.2 log reduction in 0% and 7% alc/vol beers, respectively. Thermal assisted PEF (43 ˚C≤T≤53˚C, 45 kV/cm electrical field intensity, 46 pulses, 70 μs) caused at least an additional 0.7, 2.1 and 1.8 log reductions in the yeast spore population for 0%, 4%, and 7% alc/vol beers, respectively. The lightstruck character formation assessment of nine PEF treated beers revealed that dark beers were more appropriate for PEF treatment. Optimization of the processing conditions is recommended to avoid the development of the lightstruck character. Regarding the specific energy requirements for 15 PU pasteurization, HPP required the least energy (77.4 kJ/L), followed by PEF (192.23 kJ/L) and thermal processing (188.8 kJ/L), and lastly TS (2612.1 kJ/L), which required much more energy. Lastly, environmental electron scanned microscopy (eSEM) observation of untreated, thermally treated (60°C, 10 min) and HPP-treated (600 MPa, 5 min) S. cerevisiae spores (free and inside ascus) revealed various level of disruption in the spore wall and the release of intracellular components from the spore core.