Lactose crystallization in drum dried, freeze dried and spray dried skim milk powders

Abstract: Crystallization of amorphous lactose during storage at high relative humidity and high temperatures has been found to impair the quality of skim milk powders. This is related to the glass transition of lactose. Above a certain temperature (glass transition temperature) amorphous lactose transfers through the rubbery zone into a crystalline state due to increased molecular mobility and as a result of water plasticization. During this phase transition the chemical and physical properties of lactose alter which for example results in increasing rates of the Maillard reaction and losses especially of one essential amino acid- lysine. Other than relative humidity and temperature during storage, the crystallization of lactose in milk powder is assumed to be affected by the chosen drying method. In this work skim milk powders were produced by a spray drier, a freeze drier or a drum drier. The powders obtained by each drying method were analysed, i.e. the glass transition temperature, water activity and water content were determined. Thereafter the powders were stored at two different conditions (RH 33%, 20°C and RH 53%, 30°C) for six weeks. During storage the powders were analysed after one, two, four and six weeks. Relations between the glass transition temperature, water content and water activity for each powder were established and compared in between the different drying methods. It could be concluded in this work that the glass transition temperature, determined by differential scanning calorimetry, of freeze dried skim milk powder after manufacturing was significantly higher compared to that of spray dried or drum dried skim milk powders. All skim milk powders obtained after manufacturing were in an amorphous state which implies that the process parameters used are suitable for drying skim milk. During storage of skim milk powders (drum dried, spray dried and freeze dried) at RH 33% and 20°C lactose stayed in the amorphous state throughout a storage time of six weeks. However, during storage at RH 53 % and 30°C lactose crystallized already after one week or earlier. The results of this work show which storage conditions that are most suitable to choose for a consumer of skim milk powder containing products in order to maintain the quality of the powder.