Discipline: sensory; Keywords: pasture, total mixed ration, sensory, volatile, whole milk powder.
Whole milk powder is an important product that is often exported to be reconstituted and used for various purposes. The spray drying to obtain the product enables milk to be easily transported and stored for extended periods of time. However, the spray drying process can also facilitate oxidative changes as the high fat content is exposed to elevated temperatures, resulting in reduced shelf life due to off-flavour development. Moreover, whole milk powder can also be subjected to extreme temperature fluctuations during transport and storage, further affecting oxidative stability. Therefore it is important to study both the total and the free fatty acids, to understand the potential susceptibility of dairy products to lipid oxidation and to know the contents of specific free fatty acids that can directly contribute to flavour. Milk also contains various natural oxidants and antioxidants that can be affected by feeding system, but to date very little research has been published on the susceptibility of milk or whole milk powder to lipid oxidation dependent on the feeding system. The aim of the study by Dr H J Clarke and colleagues was thus to correlate the volatile compounds with the sensory attributes of whole milk powder to appreciate the effect of lipid oxidation on sensory perception. Lipid oxidation compounds can adversely affect the sensory perception of whole milk powder by imparting rancid, metallic, and ‘painty’ tastes. The results of their study were published in the Journal of Dairy Science, Volume 104 of 2021, page 10654 to 10668, the title being Effect of bovine feeding system (pasture or concentrate) on the oxidative and sensory shelf life of whole milk powder.
Whole milk powder derived from milk produced by cows maintained on a pasture diet (grass and grass-clover mix) versus a non-pasture diet [total mixed ration (TMR); concentrates and silage] were stored at temperatures 21°C (ambient storage) and 37°C (accelerated storage) and analyzed for volatile compounds and sensory attributes every two months over six months. Thirteen volatile compounds originating from lipid oxidation were chosen to track the volatile profile of the whole milk powder during storage. Colour, composition, total fatty acid, and free fatty acid profiling were also carried out.
Significant variations in the concentrations of 14 fatty acids were observed in whole milk powder based on diet. Concentrations of free fatty acids increased in all sample types during storage. Similar trends in sensory attributes were observed with an increase in ‘painty’ attributes. Buttery/toffee attributes were found to be more closely correlated with TMR whole milk powder. Those whole milk powders derived from pasture diets were found to be more susceptible to lipid oxidation from a volatile perspective, particularly in relation to aldehyde development, which is likely due to increased concentrations of conjugated linoleic acid (CLA) and α-linolenic acid found in the samples.
The main finding from this study was that the bovine feeding system pasture versus non-pasture (TMR) significantly affected the total fatty acids, free fatty acids, volatile profile, and sensory attributes of whole milk powder. Pasture-derived whole milk powders were best correlated with creamy colour, dairy sweet aroma, and hay-like attributes, whereas TMR-derived whole milk powder was best correlated with white colour and buttery/toffee and ‘painty’ attributes. Buttery/toffee attributes were found to be more closely correlated with TMR whole milk powder. Increases in many of the volatiles studied were evident during storage at both 21°C and 37°C, with some compounds peaking at four months and then plateauing or decreasing slightly by six months, likely due to degradation exceeding formation. Pasture-derived whole milk powder was found to be more susceptible to lipid oxidation from a volatile perspective, particularly in relation to aldehyde development, possibly due to increased concentrations of CLA and α-linolenic acid. Pleasant attributes, possibly associated with Maillard reaction products were perceivable in the whole milk powder at the beginning of the study but became masked by lipid oxidation compounds with off-flavours by four months. Correlations were made between concentrations of some attributes, but it is unlikely that a single compound was responsible for these attributes. Regardless of this, however, the recommended shelf life for whole milk powder once opened was less than four months from a sensory perspective.