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Abstract
The intention of this study was to investigate the inactivation of endospores by a combined thermal and pulsed electric field (PEF) treatment. Therefore, self-cultivated spores of Bacillus subtilis and commercial Geobacillus stearothermophilus spores with certified heat resistance were utilized. Spores of both strains were suspended in saline water (5.3 mS cm−1), skim milk (0.3% fat; 5.3 mS cm−1) and fresh prepared carrot juice (7.73 mS cm−1). The combination of moderate preheating (70–90°C) and an insulated PEF-chamber, combined with a holding tube (65 cm) and a heat exchanger for cooling, enabled a rapid heat up to 105–140°C (measured above the PEF chamber) within 92.2–368.9 μs. To compare the PEF process with a pure thermal inactivation, each spore suspension was heat treated in thin glass capillaries and D-values from 90 to 130°C and its corresponding z-values were calculated. For a comparison of the inactivation data, F-values for the temperature fields of both processes were calculated by using computational fluid dynamics (CFD). A preheating of saline water to 70°C with a flow rate of 5 l h−1, a frequency of 150 Hz and an energy input of 226.5 kJ kg−1, resulted in a measured outlet temperature of 117°C and a 4.67 log10 inactivation of B. subtilis. The thermal process with identical F-value caused only a 3.71 log10 inactivation. This synergism of moderate preheating and PEF was even more pronounced for G. stearothermophilus spores in saline water. A preheating to 95°C and an energy input of 144 kJ kg−1 resulted in an outlet temperature of 126°C and a 3.28 log10 inactivation, whereas nearly no inactivation (0.2 log10) was achieved during the thermal treatment. Hence, the PEF technology was evaluated as an alternative ultra-high temperature process. However, for an industrial scale application of this process for sterilization, optimization of the treatment chamber design is needed to reduce the occurring inhomogeneous temperature fields.
Highlights
Based on the consumers’ demand for fresh, durable and safe foods it is obligatory for the food industry to preserve their products at the best
A innovative preservation method is the application of pulsed electric fields (PEF), which might by suitable for sterilization of liquid food (Siemer et al, 2014a,b)
The tested flow rates of 4.8 and 7.1 l h−1 had nearly no impact on the residence time distribution (Figure 3) and 7 min was selected as sampling time after parameter or medium change was done
Summary
Based on the consumers’ demand for fresh, durable and safe foods it is obligatory for the food industry to preserve their products at the best. PEF treatment can principally be a non-thermal process, an energy dependent increase in temperature of the treated medium will occur (Van Loey et al, 2001). This phenomenon is termed joule effect (Spilimbergo et al, 2003). A great advantage of the PEF technology is that thermal impact on foods can be reduced This may lead to fresh-like products with more natural color, texture, taste, and a higher concentration of secondary plant products while the same shelf life can be obtained. Only a limited inactivation of enzymes is achievable with this treatment, this problem can be solved by a combination of PEF and other enzyme-inactivating processes (Van Loey et al, 2001; Buckow et al, 2012; Meneses et al, 2013)
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