Morphological and mechanical properties of dried skimmed milk and wheat flour mixtures during storage R. Al Mahdi, A. Nasirpour, S. Banon, J. Scher, S. Desobry ⁎ Laboratoire de Science et Génie Alimentaires, Ecole Nationale Supérieure d'Agronomie et des Industries Alimentaires, 2, avenue de la forêt de Haye, B.P. 172, 54 505, Vandoeuvre-lès-Nancy cedex, France Received 4 March 2004; received in revised form 29 September 2005 Available online 10 March 2006 Abstract Five mixtures of dried skimmed milk (DSM) and wheat flour (WF) were prepared and stored for 45days at 20°C and various relative humidities (RH) from 0% to 85%. Morphological properties, particle size distributions and mechanical properties were studied before and during storage. Scanning electronic microscopy (SEM) showed that samples stored at high RH (59–85%) agglomerated and formed a compact mass of powder due to caking phenomenon. The caking intensity increased with storage relative humidity and was expressed as the maximum force (F max. ) calculated from force/compression curves. DSM powders showed stick-slip behaviour during deformation and, moreover, this phenomenon increased with the DSM ratio in the mixture (up to 20 ± 1N for 100% DSM) and disappeared with storage relative humidity. © 2006 Elsevier B.V. All rights reserved. Keywords: Dried skimmed milk; Wheat flour; Storage; Stick-slip; Caking 1. Introduction Food powders represent a large variety of powder materials that differ in their chemical composition and physical character- istics. In industry, these powders can be used as primary, intermediate or final products. During their lifetime, they will be subjected to a wide range of temperatures and relative humidities associated with extraction, mixing, conditioning, storage and transport. It is extremely important and difficult to preserve the quality of food powders because they are susceptible to various changes such as moisture uptake, softening, browning, compac- tion, collapse or caking. Flour and dairy powders are important ingredients that are used for dry infant foods and are significant examples of food powder processing where every mixing and storage operation must be carefully controlled. Indeed, cohesion forces between particles appearing during processing or storage can have a dramatic effect on powder mechanical behaviour. For example, depending on the environmental conditions, cohesion forces prevent powders from flowing by forming stable bonds between particles and lead to caking. It was shown that particle agglomeration could be influenced by powder particle size [1–3] as well as by moisture content [4–7]. The aim of the present work was to study the influence of the relative humidity on the morphological and mechanical properties of different mixtures made up of dried skimmed milk (DSM) and wheat flour (WF) during storage at 20°C. 2. Materials and methods 2.1. Materials Dried skimmed milk (DSM) intended for infant foods was supplied from BBA Lactalis (Belgium). Wheat flour (WF) was Powder Technology 163 (2006) 145 – 151 www.elsevier.com/locate/powtec ⁎ Corresponding author. Tel.: +33 3 83 59 58 80; fax: +33 3 83 59 58 04. E-mail address: stephane.desobry@ensaia.inpl-nancy.fr (S. Desobry). Table 1 Composition of dried skimmed milk before storage Component g/100 g dried basis Lactose 53.84 ± 0.04 Total protein: 36.59 ± 0.15 Caseins 27.88 ± 0.01 Soluble proteins 8.71 ± 0.02 Non-proteinic nitrogen 0.34 ± 0.01 Fat content 0.97 ± 0.021 Ash 7.98 ± 0.03 0032-5910/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2006.01.012