Ultrafiltration: Impact of process temperature (7 and 50°C) on process performance and protein beverage physical, chemical, and sensory properties.

Our objectives were to determine the impact of ultrafiltration (UF) of skim milk at 7 and 50°C on UF processing, lactose removal, mineral partitioning, and skim milk retentate physical, chemical, and sensory properties at 3 (3.4 7.5, and 10.5%) protein concentration with 2 different heat processing treatments high temperature short time (HTST) pasteurization and autoclave). Pasteurized skim milk was split into 2 portions and the 7°C UF processing run was done on one day and the 50°C UF processing run was done on the next day. Skim milk was ultrafiltered and diafiltered at 7 and 50°C and as permeate was removed, deionized water at 7 or 50°C was added in an equal amount by weight as permeate removed to maintain constant protein concentration in the retentate during UF until 98% or more of lactose and low molecular weight soluble milk components were removed. All skim milk-based beverage bases from the 7 and 50°C UF of skim milk were HTST (78°C for 15 s) processed or autoclaved (116°C for 6 min). The physical, chemical, and sensory properties of all treatments were measured. This process was replicated twice with a new batch of pasteurized skim milk in a different week with the 7 and 50°C UF processing runs ran in reverse order. Overall, lactose-free skim milk at 3.4, 7.5, and 10.5% protein produced by UF with DF, was more bland, more white and less heat stable (i.e., stable to retorting but not direct steam injection at 142°C for 2 to 3 s) than skim milk based on both sensory scores and instrumental measures. A 98 to 99% removal of lactose from skim milk was achieved (final lactose concentration <0.06 g/100g) with a diafiltration ratio of water to milk of about 4 to 1 was used at both 7°C and 50°C. The processing time to achieve that lactose removal from the same starting weight of milk was about twice as long when filtering at 7°C than 50°C because of the lower flux (23 versus 48 kg/m²/h). The continuous DF at constant protein concentration maintained constant flux for a processing time of 4 and 8 h at 50 and 7°C, respectively. The final freezing point of the lactose and soluble mineral reduced milk was close to that of water (-0.015°C versus -0.525°C for skim milk) and the pH of the lactose-free milk at 20°C increased from about 6.5 for skim to about 7.33 and 7.46 for UF/DF skim milk at 7 and 50°C, respectively. Removal of compounds that absorb light (in the range of 360 to 500 nm) from milk in the permeate, increased light reflectance and whiteness and decreased yellowness relative to the starting skim milk.