# Influence of Ultrasonic Pretreatment on Deliquoring Properties in Expression of Carrots

Eiji Iritani $ ^{1} $ , Nobuyuki Katagiri $ ^{1} $ , Yuta Wakamatsu $ ^{1} $ and Jun-Hyung Cho $ ^{2} $

$ ^{1} $ Department of Chemical Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya-shi, Aichi 464-8603, Japan

$ ^{2} $ Department of Paper Science and Engineering, Kangwon University, Chunchon 200-701, Korea

Keywords: Expression, Carrot, Ultrasonic Irradiation, Cake Moisture Content, Heating Effect, Pretreatment

The effect of the pretreatment of ultrasonic irradiation on the deliquoring properties in expression was investigated by using carrots as an example of vegetables. The results indicated that the ultrasonic pretreatment produced a noticeable improvement in deliquoring behaviors such as the deliquoring rate and cake moisture content due to the physical damage of cell tissue, although the carrots were maintained in the original shape before and after ultrasonic irradiation. The time variation of the cake moisture content during the course of expression was well described by the three-stage creep model combined with the modified Terzaghi model. It was necessary to heat carrots above $ 50^{\circ} $ C during ultrasonic irradiation in order to attain a desired effect in expression since carrots lost cell turgor pressure rapidly through $ \beta $ -elimination reaction occurring at about $ 50^{\circ} $ C. The ultrasonic pretreatment was well evaluated from the viewpoint of the ultrasonic energy dissipated in the sample. The cake moisture content after 24 h from the beginning of expression dramatically decreased from 69 to 47% by increasing the pressure from 0.5 to 10 MPa, indicating that the cake consisting of treated carrots behaved as the compressible material in the expression operation.

## I ntroduction

Deliquoring is a crucial issue in the food industry and has been widely used in preserving food, food waste treatment, dry food manufacturing, and boosting yield in juice production (Gallego-Juárez et al., 2007). The methods for deliquoring may be generally classified into two main types: mechanical and thermal deliquoring methods. Whilst thermal deliquoring such as drying and evaporation can remove any kind of moisture from the product, the major drawback is its high energy requirement. Instead, mechanical deliquoring such as expression and centrifugation can reduce energy consumption dramatically compared to thermal deliquoring, and thus it is a potential method as energy conservation technology. Nevertheless, mechanical deliquoring is generally unable to remove the moisture strongly attached to solids and the moisture contained in solids. Particularly, it is often difficult to dehydrate vegetables because of the existence of hard plant's cell walls. The high consolidation pressure is required to destroy such hard cell walls and to remove as much of the moisture as possible from vegetables: for instance, the pressure in the range 1–5 MPa for carrots (Grimi et al., 2010). Although consolidation mechanisms due to the expression of cellular materials are extremely complicated, a large number of studies (Buttersack, 1994; Kawasaki et al., 1996; Lanoisellé et al., 1996; Rebouillat et al., 1996; Kamst et al., 1997; Schwartzberg, 1997; Christensen and Keiding, 2007; Venter et al., 2007) have so far been reported with the consolidation model (Shirato et al., 1967, 1974) developed for soils and mineral materials as a springboard.

Suitable sample conditioning processes prior to deliquoring are frequently conducted to yield higher performance in deliquoring of cellular materials by damaging the tissue. So far, several treatments have been put forward as the pretreatment or in combination: ultrasonic irradiation (Gallego-Juárez et al., 2007; García-Pérez et al., 2009; Ozuna et al., 2011; Ruiz-Hernando et al., 2013) and microwave (Hu et al., 2006; Yu et al., 2009) in mechanical separation and drying, thermal treatment (Ruiz-Hernando et al., 2013), electric field (Chen et al., 2011), pulsed electric field (Bouzrara and Vorobiev, 2003; Grimi et al., 2007) and freezing-thawing (Kawasaki et al., 1996; Grimi et al., 2010) in mechanical separation, and radio frequency (Cohen and Yang, 1995) in drying. Among them, ultrasonic irradiation is thought to be a tool with a great potential as the pretreatment for cellular materials and applied to several fields such as osmotic dehydration (Cárcel et al., 2007), extraction process (Riera et al., 2004), sludge disintegration (Erden and Filibeli, 2010), and so forth. It is expected that the effect of ultrasonic irradiation is largely dependent on the process variables such as sample temperature, ultrasonic power, and sonication time.

In the present article, carrot deliquoring due to expression is examined, and the effect of ultrasonic irradiation as the pretreatment on deliquoring behaviors in expression is investigated. The existing consolidation model is applied in order to elucidate the consolidation kinetics in expression of carrots pretreated by ultrasonic irradiation. The pretreatment