Water plasticization led to depression of the glass transition causing significant changes in the physicochemical and crystallization properties in storage of lactose and lactose/protein (3:1) mixtures. Glass transition (T g ) and crystallization temperatures (T cr ) were determined using differential scanning calorimetry. Whey protein isolate (WPI), albumin, and gelatin increased the T g of dry powders; when Na-caseinate was used, a decrease was observed. In the presence of proteins and water, a decrease of T g at a w Յ Յ Յ Յ Յ 0.23 was observed. At a w Ն Ն Ն Ն Ն 0.33, proteins increased the T g . In the anhydrous state, T cr decreased in the presence of proteins possibly because of browning. WPI, Nacaseinate, albumin, and gelatin delayed lactose crystallization in humidified samples, with albumin and gelatin delaying it more than WPI at all storage humidities. Temperature difference between an observed instant crystallization and glass transition (T cr to T g ) was larger for humidified samples containing proteins than for lactose. Various proteins and water affect crystallization behavior of amorphous lactose differently in spray-dried powders. This should be considered in evaluating sugar crystallization properties in food products including dairy powders.

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Water sorption properties, effects of proteins on glass transition temperature, and time-dependent lactose crystallization of spray-dried lactose and lactose in lactose/WPI (3:1), lactose/Na-caseinate (3:1), lactose/albumin (3:1), and lactose/gelatin (3:1) mixtures were investigated. Brunauer-Emmett-Teller (BET) and Guggenheim-Anderson-de Boer (GAB) models were used to model water sorption. Lactose/protein mixtures sorbed high amounts of water at low relative vapor pressure (RVP) up to 23.1%. Above 23.1% RVP levels, water sorbed by pure lactose was higher, up to 44.1% RVP, except in the case of the lactose/gelatin mixture. Lactose/ gelatin also sorbed a high amount of water at 33.2% RVP. Loss of sorbed water resulting from crystallization of amorphous lactose was observed. Crystallization of pure lactose and lactose crystallization in lactose/protein mixtures occurred at RVP Ն Ն Ն Ն Ն 44.1% within 24 h. After crystallization at RVP Ն Ն Ն Ն Ն 54.5%, water contents remained higher for lactose/protein mixtures than for pure lactose. The rate of lactose crystallization was less in all lactose/protein mixtures than was observed for pure lactose. WPI had the lowest effect on lactose crystallization. Crystallization occurred most slowly in lactose/gelatin mixtures. Both GAB and BET models fitted to water sorption data up to 0.441 a w . It seems that different proteins interact with lactose differently. Water sorption and time-dependent lactose crystallization of lactose/protein mixtures have important consequences to processing and storage behavior of lactose-protein based products.

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