Optimization of co-current spray drying process of sugar-rich foods. Part I—Moisture and glass transition temperature profile during drying Vinh Truong a, * , Bhesh R. Bhandari b , Tony Howes c a Faculty of Food Science and Technology, The University of Agriculture and Forestry, HCM city, Vietnam b School of Land and Food Sciences, The University of Queensland, St. Lucia QLD 4072, Australia c School of Engineering, The University of Queensland, St. Lucia QLD 4072, Australia Received 2 February 2004; accepted 7 October 2004 Available online 2 December 2004 Abstract A steady state mathematical model for co-current spray drying was developed for sugar-rich foods with the application of the glass transition temperature concept. Maltodextrin–sucrose solution was used as a sugar-rich food model. The model included mass, heat and momentum balances for a single droplet drying as well as temperature and humidity profile of the drying medium. A log– normal volume distribution of the droplets was generated at the exit of the rotary atomizer. This generation created a certain num- ber of bins to form a system of non-linear first-order differential equations as a function of the axial distance of the drying chamber. The model was used to calculate the changes of droplet diameter, density, temperature, moisture content and velocity in association with the change of air properties along the axial distance. The difference between the outlet air temperature and the glass transition temperature of the final products (DT) was considered as an indicator of stickiness of the particles in spray drying process. The cal- culated and experimental DT values were close, indicating successful validation of the model. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Glass transition temperature; Mathematical modeling; Optimization; Maltodextrin; Sucrose; Spray drying; Stickiness; Sugar-rich foods 1. Introduction Spray drying of sugar-rich foods is difficult due to the presence of high content of sugars and organic acids. These compounds exhibit sticky behavior during spray drying. Stickiness is a term used to describe the phenom- ena of particle–particle cohesion and particle–wall adhe- sion in the spray drying process. Stickiness depends not only on the properties of materials but also on the inlet variables applied in a spray drying system. Deposition is another phenomenon in spray drying. It relates to the stickiness and the design of a dryer system. Theoreti- cally, there should no deposition problem if the size of the dryer chamber is large enough. Because of the eco- nomic factors, the size of the dryer needs to be limited to a suitable range. Therefore, stickiness and deposition need to be solved by other methods rather than by using a very large dryer chamber. Additives such as glucose syrups or maltodextrin have been used to produce physical changes in the prod- uct, consequently reducing the stickiness and wall depo- sition in spray drying. Glucose syrups have been used for blackcurrant (Zaleski, Lipowska, & Kuszlik, 1968) and orange juice (Brennan, Herrera, & Jowitt, 1971) and maltodextrin has been used for different sugar-rich foods such as orange (Gupta, 1975), blackcurrant 0260-8774/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.10.017 * Corresponding author. Tel.: +84 08 8975614; fax: +84 08 8960713. E-mail address: vinhthao@hcmc.netnam.vn (V. Truong). www.elsevier.com/locate/jfoodeng Journal of Food Engineering 71 (2005) 55–65