food and bioproducts processing 88 (2010) 133–137 Contents lists available at ScienceDirect Food and Bioproducts Processing journal homepage: www.elsevier.com/locate/fbp A new physical pretreatment of plum for drying M.H. Jazini * , M.S. Hatamipour Department of Chemical Engineering, Isfahan University, Isfahan, Iran abstract In this paper a new physical pretreatment of plums, consists of piercing them by a thin needle, is proposed to increase the rate of drying. The effect of physical pretreatment on drying time was compared with chemical pretreatment that consists of dipping of plums in hot NaOH solution (1%). Drying experiments were carried out in a convective laboratory dryer at 85 ◦ C and 0.81 m/s air velocity. It was observed that pierced plums were dried faster than chemically pretreated plums. After 480 min moisture ratio of pierced sample was 0.07 while for the chemical method it was 0.25. The moisture ratio at any time was compared with seven different mathematical models and the best model was determined according to the best agreement. Accordingly, two-term exponential model for moisture ratio is found to be superior to the other proposed models. The effective diffusivity was found to be 5.471 × 10 -9 m 2 /s for chemically pretreated and 1.016 × 10 -8 m 2 /s for physically pretreated plums. © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Plum; Physical pretreatment; Drying; Mathematical modeling 1. Introduction Plum is a source of essential nutrients, vitamins and minerals. Drying of plums makes them available at any time and avoids them from perishing. In addition, dried plums are easy to be transported, packed and shipped. Consumption of plums is becoming widespread, and it is necessary to enhance produc- tion rate and develop a new industrial drying process. Plums have a waxy layer in their outer surface that is the main bar- rier to moisture loss. The main challenge in drying of plums is to reduce drying time by pretreatment of plums to decrease the effect of waxy layer. In this manner, extensive attempts were done and some pretreatments were proposed. Goyal et al. (2007) dried untreated, blanched and dipped plums in potas- sium meta bisulphate solution (KMS) in a tunnel dryer. Tarhan (2007) applied eight pretreatment combinations for drying of plums and reported that the best pretreatment is dipping in NaOH solution (1%). Menges and Ertekin (2006) dried plums which were dipped into 2% NaOH solution in order to examine the effect of pretreatment. Sacilik et al. (2006) blanched plums in hot water of 80 ◦ C for 2 min and rinsed with tap water at room temperature immediately to increase the water perme- ability of the skin. Doymaz (2004) pretreated plums in alkali solution containing ethyl oleate. To emphasize on the role of skin on the drying of plum, Sabarez and Price (1999) com- ∗ Tel.: +98 0 311 793 2172; fax: +98 0 311 793 2170. E-mail address: m.h.jazini@gmail.com (M.H. Jazini). Received 10 February 2009; Received in revised form 30 May 2009; Accepted 22 June 2009 pared effective diffusivity of moisture when plums were dried with or without skin. In all of mentioned works, chemical or thermal pretreatment was used, and it is found that chemi- cal pretreated plums dried faster than blanched or untreated ones. There are a few attempts to apply physical pretreatments. A physical pretreatment, consists of superficial abrasion of the plums peel, is proposed by some researchers and com- pared with chemical method in which the plums were dipped into solution of ethyl oleate (Cinquanta et al., 2002; Matteo et al., 2003, 2002). This physical method was found to be more effective than chemical one. In this paper, a new physical pretreatment of plums that consists of piercing of plums by a thin needle, is proposed in order to enhance moisture loss and its influence on reducing drying time was examined. In addition drying curve is pre- sented by a suitable mathematical model. Finally, effective moisture diffusivity of physically and chemically pretreated samples was reported. 2. Materials and methods Samples of Uryani plums at commercial maturity were pur- chased from local market in Isfahan, Iran. Generally, plums of approximately uniform size (average diameter and weight of 0960-3085/$ – see front matter © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.fbp.2009.06.002