E350 JOURNAL OF FOOD SCIENCE—Vol. 70, Nr. 5, 2005 Published on Web 6/13/2005 © 2005 Institute of Food Technologists Further reproduction without permission is prohibited E: Food Engineering & Physical Properties JFS E: Food Engineering and Physical Properties Crystallization Kinetics and X-ray Diffraction of Crystals Formed in Amorphous Lactose, Trehalose, and Lactose/Trehalose Mixtures SONG MIAO AND YRJÖ H. ROOS ABSTRACT: Effects of storage time and relative humidity on crystallization kinetics and crystal forms produced from freeze-dried amorphous lactose, trehalose, and a lactose/trehalose mixture were compared. Samples were exposed to 4 different relative water vapor pressure (RVP) (44.1%, 54.5%, 65.6%, 76.1%) environments at room temperature. Crystallization was observed from time-dependent loss of sorbed water and increasing intensities of peaks in X-ray diffraction patterns. The rate of crystallization increased with increasing storage humidity. Lactose crystallized as - lactose monohydrate, -anhydrous, and anhydrous forms of - and -lactose in molar ratios of 5:3 and 4:1 in lactose and lactose/trehalose systems. Trehalose seemed to crystallize as a mixture of trehalose dihydrate and anhydrate in trehalose and lactose/trehalose systems. The crystal forms in a mixture of lactose and trehalose did not seem to be affected by the component sugars, but crystallization of the component sugars was delayed. Time-dependent crystal- lization of lactose and trehalose in the lactose-trehalose mixture could be modeled using the Avrami equation. The results indicated that crystallization data are important in modeling of crystallization phenomena and predicting stability of lactose and trehalose-containing food and pharmaceutical materials. Keywords: crystallization, lactose, trehalose, crystal form, X-ray diffraction Introduction C rystallization of sugars is of practical importance in food and pharmaceutical industries (Hartel and Shastry 1991; Price and Young 2004). Crystallization of amorphous sugars in foods may en- hance both physical and chemical deterioration (White and Cake- bread 1966; Berlin and others 1968; Saltmarch and others 1981; Hartel and Shastry 1991; Roos and Karel 1992; Miao and Roos 2004), and the changes in the degree of crystallinity of lactose in dairy systems often affects their quality (Aguilar and Ziegler 1994; Roos and others 1999). Because of its importance in food formulations and pharmaceutical systems, time-dependent crystallization of amorphous lactose has been extensively studied (van Kreveld and Micheals 1965; Twig and Nickerson 1968; Jelen and Coulter 1973; Nickerson and Moore 1974; Thurlby, 1976; Pencosat and Junk 1980; Roos and Karel 1991; Arvanitoyannis and Blanshard 1994; Buckton and Darcy 1995; Jouppila and others 1997, 1998; Biliaderis and oth- ers 2002; Haque and Roos 2004). The crystallization rate of lactose has been clearly demonstrated to be affected by temperature, rel- ative humidity, water content, and the presence of other materials during processing and storage. There are different methods available for studying crystallization of amorphous sugars. Crystallization kinetics at constant relative humidity can be investigated by monitoring time-dependent chang- es in sorbed water contents during storage and observed differences resulting from crystallization (Iglesias and Chirife 1978; Vuataz 1988; Lai and Schmidt 1990; Jouppila and Roos 1994; Jouppila and others 1998; Kedward and others 2000), by measuring decreasing amounts of heat released by crystallization based on isothermal and non-iso- thermal differential scanning calorimetry (Roos and Karel 1992; Ar- vanitoyannis and Blanshard 1994; Kedward and others 1998, 2000; Mazzobre and others 2001, 2003), and by analyzing increasing inten- sities and areas of peaks of X-ray diffraction (XRD) patterns using X- ray diffractometry (Marsh and Blanshard 1988; Roulet and others 1988; Drapier-Beche and others 1997; Jouppila and others 1997, 1998; Corrigan and others 2004). Microscopy and infrared spectros- copy are also used to study sugar crystallization of amorphous sys- tems (Akao and others 2001; Mazzobre and others 2003). Among these techniques, XRD is a practical technique for identifying crystal forms and following or characterizing rates of crystallization of com- ponents especially when the systems are complicated, such as sys- tems with several crystallizing components. Amorphous lactose may crystallize into several crystal forms: -lac- tose monohydrate (Fries and others 1971; Aguilar and Ziegler 1994), anhydrous -lactose (Buma and Wiegers 1967; Würsch and others 1984), stable and unstable anhydrous -lactose (Buma and Wiegers 1967), and anhydrous crystals with - and -lactose in molar of 5:3 and 4:1 (Bushill and others 1965; Simpson and others 1982; Briggner and others 1994; Jouppila and others 1997, 1998). The amount of anomeric forms in sugar systems is affected by mutarotation, which is depen- dent on temperature (Roetman and Buma 1974). Hartel and Shastry (1991) pointed out that other factors in addition to temperature may also affect an equilibrium ratio of anomeric forms. Such factors include concentration, pH, storage relative humidity, and the presence of oth- er compounds. The presence of anomers and tautomers of a sugar may also retard crystallization, as would any foreign sugars (Thompson and Wolfrom 1962; Shallenberger and Birch 1975; Hartel and Shastry 1991; Jouppila and others 1998). Trehalose has been reported in many studies as an effective cry- oprotectant for stabilization of enzymes and biomaterials (Crowe and others 1984; Colaco and others 1992; Uritani and others 1995). Recently, it has also been used as a new ingredient in the food in- dustry (Roser 1991; Schiraldi and others 2002), especially in lactose- MS 20040755 Submitted 11/17/04, Revised 2/28/05, Accepted 3/23/05. The authors are with Dept. of Food and Nutritional Sciences, Univ. College Cork, Ireland. Direct inquiries to author Roos (E-mail: yrjo.roos@ucc.ie ). jfsv70n5pE0350-0358ms20040755-DW.P65 6/13/2005, 11:26 AM 350