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.

Popular Abstract

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.

During storage of skim milk powder at a high relative humidity or high temperature of the air, the sugar in milk, called lactose, can undergo a transition from a disordered, instable state with high viscosity (amorphous lactose) into a more ordered, stable state (crystalline lactose). During crystallization amorphous lactose in skim milk powder is exposed to either an increase in temperature or an increase in water content, which lowers the viscosity of the amorphous state and lactose ends up in something called the rubbery state, where the molecular mobility of the material increases. In the rubbery state the rate of deteriorative reactions can increase and in this state lactose is prone to react especially with one essential amino acid present in milk –lysine. When lactose transitions from an amorphous state into a rubbery state the glass transition takes place and this happens at a certain temperature - the glass transition temperature. The end products that emerge when lactose reacts with lysine are connected to several health aspects. Skim milk powders in particular are very rich in these end products which seems to be connected to the drying method used. However, today there is a lack in knowledge about the transition of lactose in milk powders produced by different drying methods. This work aimed to deepen the knowledge and understanding of the glass transition of lactose in different milk powders after processing and also during storage.

Skim milk powders were in this work 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 was measured, the water activity (a measure of how available the water is in a system) and water content were determined. Thereafter the powders were stored at two different conditions (relative humidity 33%, 20°C and relative humidity 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 activity and water content were established and compared in between the different drying methods.

The results of this work show that storage of skim milk powders at a relative humidity of 33% and 20°C is more favourable compared to storage at relative humidity of 53% and 30°C. This could be concluded due to the fact that lactose did not undergo glass transition and staid in an amorphous state after six weeks of storage. During storage at relative humidity 53% and 30 °C in contrast lactose crystallized after one week (or earlier) which seems not very favourable when considering the quality of the powder. It can thereby be confirm that the relative humidity and temperature of the air during storage of skim milk powders have an effect on the glass transition temperature of lactose. When comparing all three drying methods freeze drying seemed to be the best alternative for drying skim milk when considering health aspects.

The high nutritional value, high protein content and low fat content in skim milk powder makes them a very favourable ingredient in many different foods such as dry mixes, bakery products, confections, infant formulas, sports and nutrition foods, etc. Skim milk powders are today consumed in high amounts which indicates how important it is to maintain the quality of the powder during manufacturing in the industry but also during storage in the home of consumers.

• author: Bick, Annette
• supervisor: Marilyn Rayner; Ingegerd Sjöholm; Kataneh Aalaei
• organization: Food Technology and Nutrition (M.Sc.)
• course: KLT920 20142
• year: 2015
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: lactose crystallization, skim milk powder, glass transition
• language: English
• id: 5146995
• date added to LUP: 2015-03-09 17:10:46
• date last changed: 2015-05-21 11:35:51