ABSTRACT: With the use of two central composite designs, the effects of agitation rate, fractionation temperature, and resi- dence time on the thermal properties of the stearin and olein milk fat fractions were investigated. The main function of agita- tion during fat fractionation was suspending the crystal aggre- gates and enhancing the heat transfer. For the experimental con- ditions described here, crystal aggregation did not seem to be affected by agitation. The effect of fractionation temperature on the physical properties of the olein fraction was very significant. Triangle diagrams were shown to be a useful tool for monitoring and designing fractionation processes. They illustrate that oleins with similar melting properties can be produced over a range of yields of stearin, which is important from an industrial point of view. Crystallizer residence time, which influences production costs, clearly affects both stearin yield and olein melting proper- ties. For any fractionation temperature, stearin fractions with vir- tually identical melting properties and yields can be obtained over a range of olein melting properties. Manipulation of both the fractionation temperature and residence time allows the frac- tionation process to be adapted to meet changing market de- mands for fractions with different melting properties. Paper no. J10135 in JAOCS 79, 1169–1176 (December 2002). KEY WORDS: Agitation, milk fat fractionation, residence time, temperature, triangle diagram. The production of anhydrous milk fat (AMF) was a conser- vation method for the large stocks of butter produced during the 1980s in the European Community. By removing the water phase from butter, milk fat could be stored for several years without significant loss of quality. Nowadays, AMF is used in the confectionery, bakery, and ice cream industries for its sensorial properties and its marketing value as a natural in- gredient. New techniques such as fractionation, texturization, and recombination have led to new applications for AMF, es- pecially in puff pastry and cold spreadable fat blends. These processes are used to improve product quality. Fractionation is a separation process by which the fat is di- vided into different fractions, each having its own physical and chemical properties. Two types of fractionation exist: dry (or melt) fractionation and solvent fractionation (1). The lat- ter is never used on an industrial scale for milk fat owing to flavor loss but is extensively used in vegetable oil processing, e.g., for the production of cocoa butter substitutes. In dry frac- tionation, crystals are formed by controlled cooling and agi- tation. The crystals in suspension are then separated on a ro- tary drum belt filter, a filter press, a centrifuge, or by means of an emulsifier followed by a centrifugation step. Fractiona- tion of AMF was extensively reviewed by Kaylegian and Lindsay (2) and more briefly by Deffense (3). The effects of process parameters such as agitation rate, cooling rate, and fractionation temperature have been investi- gated several times. Deffense (4) discussed four factors that influence crystallization of milk fat during fractionation: oil composition, polymorphism, rate of cooling, and intersolu- bility. In addition, the technique used to crystallize milk fat (type of crystallizer, impeller, and operating conditions) can significantly affect the process (5). Several process parame- ters will be discussed in more detail. Cooling rate. For the Tirtiaux process (low agitation speed, large volume/cooling surface ratio, and minimal supercool- ing), decreased crystal size and a more uniform crystal size distribution have been observed with low cooling rates. At these cooling rates, little agglomeration took place, whereas at medium and high cooling rates, more irregular crystal agglom- erates were formed (4). In contrast, Herrera and Hartel (6) ob- tained larger, denser crystals with slow cooling rates and a more uniform crystal size distribution with higher cooling rates. This illustrates that the effect of process parameters is influenced by the working range of the experiments, the type of crystallizer, the type of impeller, and other factors. Agitation. The effect of agitation is more consistent. Sam- ples crystallized at the highest shear rates produced the small- est crystals and the narrowest particle size distribution (6,7). At low agitation, agglomeration took place, whereas at high shear rates, smaller crystals were formed because of an en- hanced nucleation rate (6,8) and breakdown of crystals by the shear forces of the impeller (5,9). Grall and Hartel (8) found the same effect at 30 and 20°C but the opposite at 15°C. This phenomenon was explained by the crystal habit: The uniform spheres formed at 15°C did not seem to be sensitive to shear forces. Patience et al. (5) investigated the effects of stirring and type of impeller on filtration properties and crystal size distribution. Breitschuh and Windhab (7) showed that higher agitation promotes cocrystallization, probably due to an enhanced heat transfer. Herrera and Hartel (6) observed that agitation had a minor influence on the induction time of the Copyright © 2002 by AOCS Press 1169 JAOCS, Vol. 79, no. 12 (2002) *To whom correspondence should be addressed at Ghent University, Faculty of Agricultural and Applied Biological Sciences, Department of Food Tech- nology and Nutrition, Coupure Links 653, B-9000 Ghent, Belgium. E-mail: bert.vanhoutte@rug.ac.be Monitoring Milk Fat Fractionation: Effect of Agitation, Temperature, and Residence Time on Physical Properties Bert Vanhoutte a, *, Koen Dewettinck a , Brecht Vanlerberghe b , and André Huyghebaert a a Department of Food Technology and Nutrition, Faculty of Agricultural and Applied Biological Sciences, Ghent University, B-9000 Ghent, Belgium, and b Aveve Dairy Products, B-8650 Klerken-Houthulst, Belgium