Impact of lactose and soluble milk salts on the heat stability of milk proteins

Our objective was to determine the separate effects of lactose, soluble mineral concentration, and added dipotassium phosphate (DKP) on the heat stability of milk protein beverages made from rehydrated milk protein concentrate (MPC) with 85% crude protein on a DM basis (MPC85). Seven batches of rehydrated (7.5% protein) liquid MPC were made from each of 2 lots of MPC85 powder. Treatment 1 was 7.5% MPC85 in deionized (DI) water. Treatment 2 was 7.5% MPC85 that was rehydrated with 4°C Jenness-Koops (JK) buffer, a mix of salts that simulate milk salts. Treatment 3 was 7.5% MPC85 with JK buffer with 4.8% (wt/wt) anhydrous lactose added. Treatment 4 was 7.5% MPC85 with a dilution of JK buffer to ∼14% of its original concentration. Treatment 5 was 7.5% MPC85 with diluted JK buffer with lactose added to 0.6% (wt/wt). Treatment 6 was 7.5% MPC85 in DI water with 0.15% (wt/wt) DKP. Treatment 7 was 7.5% MPC85 rehydrated with 4°C UF permeate. Heat stability testing was conducted using an oil bath with a 30-s hold time at 6 temperatures: 135°C, 140°C, 145°C, 150°C, 155°C, and 160°C. The composition of the solute phase of rehydrated and smoothed dried MPC85 had a large impact on the heat stability and aggregation of milk proteins caused by UHT thermal processing temperatures. When MPC85 was rehydrated in deionized water, it had the highest heat stability and the least protein aggregation, and the heat stability was not increased by the addition of DKP. The heat stability of MPC85 was decreased by rehydration in a typical concentration of milk salts (i.e., JK buffer) and was further decreased by the addition of 4.8% anhydrous lactose to the JK buffer. When the MPC85 was rehydrated in UF milk permeate (i.e., milks salts + lactose), the heat stability was comparable to the JK buffer with added lactose. Using diluted JK buffer (∼14% of the level of milk salts and lactose) in MPC85 resulted in increased heat stability that was similar to rehydration in deionized water.