performance is evaluated with varying the values of several operating parameters such as sample temperature, ultrasonic power, and sonication time. Moreover, the influence of consolidation pressure on the expression behaviors is examined.

### 1. Experimental

### 1.1 Materials

Carrot (Daucus carota ssp. sativus) was selected as a subject of the present study. Carrots of good quality were purchased in the Nagoya University cooperative, stored in a refrigerator at $ 5 \pm 1^{\circ} $ C after they were sealed in plastic films to avoid moisture loss, and used in the experiments within a week of purchase. The production areas of carrots used varied seasonally (Aichi, Aomori, and Hokkaido areas), and a set of experiments were carried out using the carrots harvested at the same area. After skin, top and bottom ends of the carrots were removed, the carrots were cut into cubes 3 mm per side by using a vegetable slicer, which can cut into a minimum of 3 mm dices, just before use. The true density of solids was measured by a pycnometer. The initial moisture content of carrots measured using an infrared-ray moisture meter (FD-720, Kett Electric Lab.) was within 87–91 wt% on a wet basis. In order to diminish the effect of impurity, the water used in the preparation of sample was ultrapure, deionized water (minimum resistivity: $ 18 M\Omega cm $ ) prepared by purifying tap water through ultrapure water systems equipped with both Elix-UV20 and Milli-Q Advantage for laboratory use (Millipore Corp.).

### 1.2 Ultrasonic pretreatment

Fifteen grams of cubic carrots were placed into a 50mL beaker, and 30mL of water was added. The beaker containing the sample was placed in a water bath incubator (BT23, Yamato Scientific Co., Ltd.) in order to keep the sample temperature constant during the ultrasonic pretreatment. The ultrasonic apparatus was an ultrasonic homogenizer (UP-200S, Dr. Hielsher GmbH) equipped with a probe with an operating frequency of 24kHz and a load power ranging from 40 to 200 W. The ultrasonic probe was submerged in the sample to a depth of 1cm above the bottom of the beaker. The sample was processed with the probe for the specified total operating time by pulsed ultrasonic irradiation in which one cycle consisted of both the operating time of 0.5s and the downtime of 0.5s in order to avoid a rise in temperature. The temperature inside of the sample was measured using a thermocouple before and after sonication, and it was confirmed that the temperature rise due to sonication was negligible. For comparison, the sample was prepared by heating in the water bath incubator without sonication. The ultrasonic power dissipated into a liquid was measured according to the method of Koda et al. (2003). The ultrasonic power I is given as

$$ I=\left(\frac{\mathrm{d}T}{\mathrm{d}t}\right)C_{\mathrm{p}}M $$

where T is the sample temperature, t is the net ultrasonic exposure time, $ C_{p} $ and M are the heat capacity and mass of water, respectively. The term dT/dt represents the rate of increase in temperature. Accordingly, the ultrasonic power I corresponding to a specified load power can be calculated from the measurement of the initial temperature rise of water under sonication by using a thermocouple.

### 1.3 Deliquoring due to expression

The expression experiments of treated and untreated carrots were conducted using the so-called compression-permeability cell (C-P cell) (Grace, 1953; Okamura and Shirato, 1955), which comprised of a cell cylinder and a piston with a cross-sectional area of $ 9.62 \, cm^2 $ , as an expression cell, as shown in Figure 1. The sample was placed in the cell cylinder and the movable piston was inserted in the cell. The liquid was then squeezed out of the sample through the top and bottom filter cloths (TRG803K, Okayama Nakao Filter Media Corp.) by exerting a mechanical load through the piston by the use of a material testing machine (SC-20H(MNS-01), Tokyo Testing Machine Inc.). The semi-solid cake pre-consolidated in the C-P cell under a pressure $ p_{1} $ of 0.1 MPa was consolidated for 24 h at constant pressures $ p_{2} $ ranging from 0.5 to 10 MPa. The time evolution of the cake thickness during constant pressure consolidation of pre-consolidated cake was measured by a dial gauge fitted on the cell cylinder. The residual moisture content in the compressed cake was measured using the infrared-ray moisture meter at the end of expression.

### 2. Results and Discussion

### 2.1 Effect of ultrasonic irradiation

Typical deliquoring behaviors due to expression of carrots with and without the pretreatment of ultrasonic irradiation are compared in Figure 2(a). Expression experiments are conducted under the condition of the consolidation pressure of 0.5 MPa for the semi-solid cake pre-consolidated under a pressure of 0.1 MPa. In the case of the treated carrots, ultrasound is irradiated for the net ultrasonic exposure time t of 15 min under the conditions of the load power P of 40 W and the sample temperature T of 50°C. The temporal variation of moisture content R of the compressed cake on the

Fig. 1 Schematic view of expression apparatus