Effect of Lactose and Protein on the Microstructure of Dried Milk

Skim milk of approximately 3% total protein was ultrafiltered to 16.2% protein and 4.1 % lactose, and then diafiltered to reduce lactose to 2.1% and 0.9%. Total protein con ten t was maintained at approximately 17%. A ponion of the sk im milk was condensed in a rising film evaporator to 14.7% protein and 15.7 % lactose . All concentrates were spray dried at 120 to 125 °C inlet air temperature and 75 to 80 oc outlet tem pe rature using a rotary atomizer in a pilot plant sp ray dryer . Moisture content of the powders were 4.7 to 6.3% and lactose content ranged from 3.1% in dia filt e red m i I k powder to 51 . 4% in the condensed mi I k powder. Scanning electron microscopy showed that powder partic les with the highest lactose content had a wrinkled su rface and dents. Powder particles with lower lactose, including ultrafiltered and diafiltered milk powders, possessed dents but had no wrinkles and were smooth. When the lactose content of the diafiltered skim milk containing 0.9 % lactose was raised to 15.6% by addition of lactose powde r and then the product was spray dried, the powder partic les showed a wrinkled surface. Intermediate lactose products, such as whey pr01ein concentrate (37.1% lactose) possessed particles with a wrinkly surface as well. The wrinkles were not as pronounced as in powders with high lactose. Permeate powder co ntaining 82.8% lactose had smooth particles with some wrinkles. Results suggest that lactose and protein influence the surface structu re of milk powder particles. Key \-Vo r ds: Spray drying, Dried milk, Microst ructure, Lactose, Protein , Partic le surface, Wrinkles , Scanning electron microscopy, Dents Initial paper received December 24, 1991 Manuscript received February 27, 1992 Direct inquiries to V. V. Mistry Telephone number: 605 688 4116 Fax number : 605 688 6065