Characterisation of spray-dried emulsions with mixed fat phases

Fat encapsulation in spray-dried protein-stabilised emulsions is known to depend on the choice of protein, the emulsion droplet size, and the melting point of the fat. However, the fat encapsulation may also depend on the fat crystal habit. Fats may crystallise in three different forms , β′ and β, of which the β-form is thermodynamically stable. The -form is obtained in rapidly cooled fats, and it can then transform into the β′-form during storage, and this crystal form is finally transformed into the β-form. In order to investigate the effect of different fat phases on the spray-dried emulsions, two solid fats were studied: fully hardened rapeseed oil (β-stable) and fully hardened palm oil (β′-stable). The solid fats were used on their own or in mixtures with rapeseed oil, in order to provide fat phases with different properties. The emulsion composition was chosen as to mimic the composition of whole milk, i.e. 40% lactose, 30% sodium caseinate and 30% fat on a dry weight basis. The dried powders were stored under dry conditions at 4 or 37 °C in order to investigate the changes in the fat crystals and surface composition of the powders with time. The surface composition was analysed using electron spectroscopy for chemical analysis. Evaluation of the data showed that surface coverage of fat varied depending on the composition of the fat phase. The ratio of lactose to protein remained constant, which implies that the fat was present as ‘islands’ on a surface composed of lactose and protein. The hardened palm oil crystallised initially in the - or β′-form (depending on the ratio of hardened fat to oil), and during storage, the crystal form gradually changed into the β′-form. In powders containing hardened rapeseed oil only the stable β-form was found, even in fresh samples. The surface coverage of fat was reduced after storage, whereas the ratio of lactose to protein at the surface remained unchanged. The emulsion droplet size in emulsions prepared at a low homogenisation pressure was considerably increased after spray-drying and reconstitution, whilst the emulsion droplet size was well preserved in emulsions prepared at high homogenisation pressure.