Original article Determination of critical storage conditions of coffee oil microcapsules by coupling water sorption isotherms and glass transition temperature Elen Cristina Frascareli, 1 Vanessa Martins Silva, 1 Renata Valeriano Tonon 1,2 * & Mı´riam Dupas Hubinger 1 1 Department of Food Engineering, School of Food Engineering, University of Campinas (UNICAMP), Campinas, SP 13083-862, Brazil 2 Embrapa Food Technology, Rio de Janeiro, RJ 23020-470, Brazil (Received 26 July 2011; Accepted in revised form 22 December 2011) Summary Coffee oil microcapsules were produced by spray-drying using as encapsulating agents gum Arabic, whey protein isolate (WPI) and mixtures of maltodextrin (MD) and WPI, at three different proportions (3:1, 1:1 and 1:3). Sorption isotherms were determined by the gravimetric method, and the glass transition temperature (T g ) was measured by differential scanning calorimetry. Microcapsules produced with gum Arabic showed the highest water adsorption and underwent a stronger water plasticising effect. At the highest water activity (0.843), this sample had a T g value of )18.3 °C, while those produced with WPI at higher proportions showed values around 25 and 30 °C. The good fit of experimental data by Guggenheim-Anderson-de Boer and Gordon–Taylor models allowed the determination of the critical storage conditions, at which powders are not susceptible to deteriorative physical changes. The samples produced with 1:1 and 1:3 of MD ⁄ WPI were the most stable ones, showing critical water activity values of 0.890 and 0.938, respectively. Keywords Coffee oil, critical storage conditions, glass transition, microencapsulation, water sorption. Introduction Coffee oil is an important coffee industry by-product, produced by mechanical extraction of roasted coffee grains. This oil presents a lipid fraction mainly com- posed by triglycerides, sterols, tocopherols and diterp- enes of the kaurene family, besides a typical coffee volatiles fraction with almost 700 compounds (Ishii, 1987; Speer & Ko¨ lling-Speer, 2006). It has several important applications, being used to avoid coffee grains agglomeration (Oliveira et al., 2005) and as a flavouring agent in the food industry, in sweets, biscuits, instant coffee, iced coffee, desserts and puddings, as well as in the cosmetic industry. However, its exposition to atmospheric air can cause aroma losses and lipid oxidation. In this context, microencapsulation has been proposed to avoid such degradation processes. Spray-drying is the most common and economic technique to produce microencapsulated food materials. It involves the conversion of liquid oils and flavours into free-flowing powders, which are easier to handle and to incorporate into dry food systems (Gharsallaoui et al., 2007). The initial step of microencapsulation is the selection of a suitable wall material. In the case of flavour and oil cores, in particular, the ideal wall material should have good emulsifying properties, be a good film former, have low viscosity at high solid levels, exhibit low hygroscopicity, release the flavour when reconstituted in a finished food product and afford good protection against oxidation (Jafari et al., 2008). Gum Arabic is largely used in the microencapsulation of oils and flavours (Bertolini et al., 2001; Fang et al., 2005; Krishnan et al., 2005). According to Madene et al. (2006), this wall material is considered ideal to the encapsulation of lipid droplets, as it fulfils the roles of both surface-active agent and drying matrix. However, its high cost, limited supply and quality variations have restricted its use, leading researchers to search for alternative materials for that purpose (Gharsallaoui et al., 2007). Maltodextrin is one of the most traditional carbohydrates used for microencapsulation, and its main advantages are its good film formation capacity and low cost. However, the big limitation of this wall material is its deficiency with respect to emulsifying properties. In this context, the use of proteins in combination with maltodextrin as encapsulating agents has been suggested as an alternative substitute for gum Arabic. The good emulsifying properties of proteins, together with the reduction on the polymeric matrix *Correspondent: Fax: +55 21 36229713; e-mail: renata.tonon@yahoo.com.br International Journal of Food Science and Technology 2012, 47, 1044–1054 1044 doi:10.1111/j.1365-2621.2012.02939.x Ó 2012 The Authors. International Journal of Food Science and Technology Ó 2012 Institute of Food Science and Technology