Storage stability of freeze-dried, spray-dried and drum-dried skim milk powders evaluated by available lysine

• Maillard reaction was monitored in skim milk powders by available lysine.
• The type of dryer and the storage variables had significant impacts.
• Humidity and temperature of storage determine the loss of available lysine.

Formation of Maillard products and the influencing factors, which are of crucial importance for both manufacturers and consumers, are still not fully understood. Thus in this study available lysine was used as a marker to monitor the extent of Maillard reactions in freeze-dried, spray-dried and drum-dried skim milk powders during 200 days of storage at highly controlled atmospheres. Storage variables included two temperatures (20 °C, 30 °C) and two relative humidities (33%, 52%). The available lysine in five replicates was quantified at pre-determined intervals by a dye-binding method using Acid-orange 12, validated in our previous work. Findings of this study show that temperature and relative humidity during storage have a profound influence on the rate of available lysine loss. Choice of the drying technology as the other investigated variable also had a significant impact. The drying process least affected the available lysine content in freeze-dried powders, followed by spray-dried and drum-dried powders. Storage at 52% relative humidity and 30 °C for 200 days led to a 39.2–45.9% decrease in the available lysine content, regardless of the drying of skim milk powder, while the powders stored at 33% relative humidity and 20 °C did not show a significant loss during the same period of time.