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Manufacturing operations of non-fat dry milk and products such as milk protein concentrate influence sensory properties, e.g. off-flavours, in dried dairy ingredients and consequently decrease consumer acceptance of products where these ingredients are used. One problem is that condensed skim milk may be stored at refrigeration temperatures for extended periods before spray drying due to transport time or lack of drying capacity. Also, spray-drying procedures may affect the physical and sensory properties of whole milk powder and whey protein concentrate. These issues formed the basis of studies by Dr C.W. Clark and colleagues which they published in the Journal of Dairy Science, Volume 99 of 2016. The title of the paper in page 9586 to 9597 is:Condensed milk storage and evaporation affect the flavor of nonfat dry milk and in page 9598 to 9610: The effect of spray-drying parameters on the flavor of nonfat dry milk and milk protein concentrate 70%.
Currently, non-fat dry milk processors have two options for milk concentration up to 30% solids: evaporation or reverse osmosis. The objective in the first study was to determine the effect of condensed milk storage and milk concentration method (evaporation vs. reverse osmosis) on the flavour of non-fat dry milk and investigate the reasons for flavour differences. In Experiment 1, skim milk was pasteurized and concentrated to 30% solids by evaporation or reverse osmosis and then either stored for 24 hours at 4°C or concentrated to 50% solids by evaporation and then spray dried immediately. To investigate the reasons for the results in Experiment 1, Experiment 2 was designed. In Experiment 2, pasteurized skim milk was subjected to one of four treatments: Control (no evaporation); heated in the evaporator without vacuum; evaporation with concentration to 30% solids, or evaporation with concentration to 40% solids. The milks were then diluted to the same solids content and evaluated. Volatile compounds were also measured during concentration in the vapour separator of the evaporator. Sensory properties were evaluated by descriptive sensory analysis and instrumental volatile compound analysis was conducted to evaluate volatile compounds.
In Experiment 1, evaporation decreased the sweet aromatic flavour and many characteristic milk flavour compounds and increased “cardboard” and cooked flavours in non-fat dry milk compared with reverse osmosis. Storage increased “cardboard” flavour and decreased the sweet aromatic flavours and vanillin concentration. Results from Experiment 2 indicated that the characteristic milk flavours and their associated volatile compounds were removed by the vapour separator in the evaporator due to the heat and vacuum applied during concentration.
These results demonstrate that off-flavours are significantly reduced when reverse osmosis is used instead of evaporation and storage of condensed milk is avoided.
The objective in the second study was to determine the effect of inlet temperature and feed solids concentration on the flavour of non-fat dry milk and milk protein concentrate. Condensed skim milk (50% solids) and condensed liquid milk protein concentrate (32% solids) were produced using pilot-scale dairy processing equipment. The condensed products were then spray dried at either 160, 210, or 260°C inlet temperature and 30, 40, or 50% total solids for non-fat dry milk and 12, 22, or 32% for milk protein concentrate in a randomized order. The experiment was replicated three times. Flavour of the non-fat dry milk and milk protein concentrate was evaluated by sensory and instrumental volatile compound analyses. Surface free fat, particle size and furosine were also analyzed.
In general, the results were consistent between the non-fat dry milk and milk protein concentrate. Increasing inlet temperature and feed solids concentration increased the sweet aromatic flavour and decreased the “cardboard” flavour and associated lipid oxidation products. Increases in furosine with increased inlet temperature and solids concentration indicated that Maillard reactions were increased during drying. Particle size increased and surface free fat decreased with increasing inlet temperature and solids concentration.
The results demonstrated that increasing inlet temperatures and solids concentration during spray drying, off-flavour intensities in non-fat dry milk and milk protein concentrate are decreased. The heat treatment effect is greater compared with the solids effect.