Effect of nitrogen flushing and storage temperature on flavour and shelf-life of whole milk powder

The objective of the study was to establish whether nitrogen flushing and colder than normal storage temperature will be favourable to flavour and shelf life of whole milk powder. This research by Dr M.A. Lloyd and colleagues was published in The Journal of Dairy Science, Volume 92 of 2009, pages 2409 to 2422. The title of the paper is: Effect of nitrogen flushing and storage temperature on flavour and shelf-life of whole milk powder.

Because whole milk powder (WMP) contains between 25 and 40% fat, it is highly susceptible to oxidation and therefore off-flavours, even rancidity. In the US the standard for shelf-life of WMP is six to nine months if stored below 27°C and 65% relative humidity, although flavour changes may occur as early as three months if kept at ambient temperature. It is obviously important to both producers and end users that storage stability should be as long as possible. Factors that have been identified which influence WMP flavour and shelf-life include initial milk quality, processing conditions, air quality, moisture content, packaging, oxygen exposure, presence of antioxidants, light exposure and storage temperature. What is also significant is that the off-flavours in WMP can carry through to the final product, for example yogurt, hot cocoa and white chocolate, resulting in decreased consumer acceptance.

In order to overcome some of these problems Dr Lloyd and colleagues evaluated in this research the influence of WMP packaging atmosphere, storage temperature and storage time on WMP shelf-life. For the packaging atmosphere air was compared to nitrogen flushing, for storage temperature 2°C was compared to 23°C, and they monitored changes over a total period of one year. The monitoring was done by chemical methods measuring solid and volatile substances in the laboratory and by trained-panel sensory analyses.

The results show that optimum shelf-life of WMP in terms of flavour (loss of "fresh" flavours and/or development of "grassy" off-flavour) was about three months, regardless of package atmosphere or storage temperature. After six months, "painty" off-flavours in WMP were detected by the sensory panel in chocolate. Nitrogen flushing greatly enhanced storage stability of WMP by preventing the development of "painty" flavour. It was concluded that packaging oxygen levels should be as low as possible to prevent lipid oxidation and off-flavours. Cooler (2°C) storage temperature also enhanced storage stability of WMP, but to a lesser extent than nitrogen flushing. It was also shown that WMP stored in nitrogen-flushed packaging for up to one year could be successfully used to manufacture milk chocolate.

Bottom line: Nitrogen-flushing when whole milk powder is packed and storing the packaged product in the fridge should extent shelf-life considerably and should prevent off-flavours.