Stability of Instant Coffee Foam by Nanobubbles Using Spray-Freeze Drying Technique

Instant coffee with stable foam is considered to be an important parameter for consumer preference and acceptability. For foam sustenance, nanoscale bubbles are more useful compared with microbubbles, due to their high specific area and high stagnation in the liquid phase (without undesirable liquid drainage). The technique that produces nanobubbles in coffee would concomitantly produce and preserve the coffee foam, the best. Spray-freeze drying (SFD) is known to be more effective for the production of instant coffee, compared with conventional spray drying (SD) and freeze drying (FD) techniques. However, its efficiency in the production of nanobubbles has not been explored. To address the issue, in the present study, SFD has been employed to produce instant coffee, and the findings have been compared with SD and FD. The coffee powder obtained with SFD produced a foam with higher stability that also comprised of nanobubbles, in contrast to SD and FD powders. The FE-SEM analysis of SFD foam showed the presence of nanobubbles in the range of 100-200 nm. When the beverage was prepared, the SFD coffee powder dissolved in water at 90 degrees C produced an excellent foam. The said foam structure was intact up to 2400 s (40 min), and lost only 89.5 +/- 2 mm of foam height, during the experiment. Thus, apart from instant coffee, a stable foam in coffee comprising nanobubbles can also be achieved through the SFD.