The effect of preheat treatment and other process parameters on the coffee stability of instant whole milk powder

Agglomerated whole milk powders were manufactured on a pilot-scale milk powder plant using different preheating conditions as well as adjustment of other process parameters. A standard coffee stability test was modified by preparing aqueous coffee solutions of varying hardness when testing the experimentally produced powders. Assaying coffee stability under hard water conditions differentiated more clearly the effects of different processing parameters, as well as providing more realistic test conditions. Higher evaporator preheat temperatures (75–120°C) and prolonged holding times (30–120 s) had a negative effect on the coffee stability of whole milk powder, as did raising concentrate total solids from 43 to 48% in the feed to the drier. Concentrate homogenisation and mode of emulsion formation had only a marginal effect on coffee stability. Optimum concentrate heating temperature was 70°C for the 5 min transmission time the concentrate took to reach the drier nozzle. Addition of lecithin during the secondary drying stage improved coffee stability. Coffee sediments contained slightly higher levels of α_s-casein than that of the original whole milk powder. However, the increase in β-lactoglobulin content of the sediment from 2.1 to 7.5% as preheat temperature was increased from 75 to 120°C was the most notable change that occurred in sediment composition. The results suggest that a combination of optimised thermal treatment as well as the attainment of the desired powder physico-chemical properties is necessary to achieve satisfactory stability of whole milk powder in coffee.