Fundamentals of spray freezing of instant coﬀee C.S. MacLeod a , J.A. McKittrick a , J.P. Hindmarsh b , M.L. Johns a , D.I. Wilson a, * a Department of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, UK b The Riddet Centre, Massey University, Palmerston North, New Zealand Received 2 November 2004; accepted 7 March 2005 Available online 11 May 2005 Abstract The freezing of single 2 mm diameter droplets of coﬀee solution in cold moving air down to 15 °C has been studied for 10– 50 wt.% coﬀee solutions in a novel temperature monitoring apparatus and in a similar device installed within a nuclear magnetic resonance (NMR) spectrometer. Heat transfer measurements and modelling conﬁrmed that droplet internal temperatures were almost uniform (low Biot number regime), which was consistent with the microstructures generated. Fundamental parameters which could be extracted from the data included nucleation temperature, degree of recalescence, freezing temperature, and extent of freez- ing and freeze concentration. The mode of nucleation (spontaneous or forced) was observed to depend on concentration, cooling rate and air temperature. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Spray freezing; Coﬀee; Nucleation; NMR 1. Introduction The manufacture of particulate products via spray drying is an established technique in the food sector, but requires the products to be stable under the temper- ature regimes employed. In contrast, spray-freeze crys- tallisation involves a liquid or solution being solidiﬁed by atomisation in a cold low humidity environment, resulting in reduced volatility and loss of aroma com- pounds, and diﬀerent microstructures. The particulates thus generated can be subsequently freeze-dried to yield stable products. The use of spray freezing for foods was proposed by Meryman (1959) as it oﬀers advantages over water removal by conventional freeze drying via increasing overall mass transfer rates. In addition, by nature of their size, small droplets oﬀer more homoge- neous temperature ﬁelds for freezing (small Biot num- bers for heat transfer), promoting uniform nucleation and microstructure development. The cost of the cold, dry gas means that this process route is likely to be pre- ferred for high value products such as pharmaceutical, biotechnological or functional material applications (e.g. Moritz & Nagy, 2002). Freeze-dried coﬀee, although more energy intensive and expensive, yields a better quality product than spray drying (Clarke & Macrae, 1987; Coste, 1992; Ratti, 2001). Mumenthaler and Leuenberger (1991) investi- gated spray freezing as an alternative route to conven- tional freeze drying of coﬀee and found that the former yielded a free-ﬂowing powder which retained more aroma and ﬂavour compounds whilst maintaining qualities of good colour, re-wetting and appearance. Furthermore, the droplet regime permits homogeneous cooling and high cooling rates, which are key factors in consistent microstructure generation (Pardo, Seuss, & Niranjan, 2002). Fig. 1(i) and (ii) shows typical temperature proﬁles for droplets of water and coﬀee solution, respectively. 0260-8774/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.03.034 * Corresponding author. Tel.: +44 1223 334 777; fax: +44 1223 334 796. E-mail address: ian_wilson@cheng.cam.ac.uk (D.I. Wilson). www.elsevier.com/locate/jfoodeng Journal of Food Engineering 74 (2006) 451–461