Enhanced expression of juice from soft vegetable tissues by pulsed electric fields: consolidation stages analysis

The expression behaviour of different soft vegetable tissues is compared. The detailed analysis of different consolidation stages is done for potato tissue. The primary consolidation Terzaghi's coefficient C-v is estimated as C-v approximate to (2.0 +/- 0.6) x 10(-2) cm(2)/s. The logarithmic type of secondary consolidation is observed and the dependency of the consolidation processes on the applied pressure is discussed. The four main time constants that characterize the consolidation curve are defined: t(0) is the time of pre-consolidation stage, t(1) is the time of intensive liquid expression during the primary consolidation stage, t(2) is the time of transition to the secondary consolidation, and t(d) is the expected 'theoretical' time of complete liquid expression. At a pressure of P = 5 bar, to t(0) approximate to 10 s, t(1) approximate to 10(3) (s,) t(2) approximate to 10(4) s, and t(d) approximate to 6 x 10(4) s (17 h). Consolidation behaviour after application of pulsed electric fields (PEF) is studied. At low degrees of PEF treatments (electric field intensity E < 300 V/cm, total time of PEF treatment T-PEF < 0.1 s), unchanged pressing behaviour at the primary consolidation stage (t < t(1)) and enhanced expression in the transitional time interval t(1) < t < t(2) was observed. It was concluded that substantial intensification of the pressing processes needs a rather high degree of PEF-induced tissue damage and low-level PEF-induced damages show notable effects only at large t > t(1), which is disadvantageous from industrial point of view. It is shown that PEF should be applied after a pre-consolidation stage. But the time at which PEF treatment starts t(PEF) has no significant effect on the juice yield for the case when t(0) < t(PEF) < t(1). An optimum value of PEF intensity E required for high intensification of juice expression at minimum electric power consumption lies in the interval of 400-600 V/cm. (C) 2003 Elsevier Science Ltd. All rights reserved.