Coalescence of Expanding Bubbles: Effects of Protein Type and Included Oil Droplets - Food Colloids: Self-Assembly and Material Science - page 369

Chapter 25
Coalescence of Expanding Bubbles: Effects
of Protein Type and Included Oil Droplets
Brent S. Murray, 1 Andrew Cox, 2 Eric Dickinson, 1 Phillip V.
Nelson 1 and Yiwei Wang 1
1 PROCTER DEPARTMENT OF FOOD SCIENCE, UNIVERSITY OF
LEEDS, LEEDS LS2 9JT, UK
2 UNILEVER RESEARCH, COLWORTH LABORATORY,
SHARNBROOK, BEDFORD MK44 1LQ, UK
25.1 Introduction
Air may be viewed as a renewable, cheap, 'zero-calorie' replacement for fat in
food products, where it can provide the desired bulk volume, texture, stability
and appearance. Air bubbles are a traditional structural component of many
foodstuffs such as mousses, ice cream, whipped toppings, etc. However, apart
from instances where the continuous phase of a food foam is solidified (as in
meringues and baked products such as bread and cakes), when compared with
emulsions and solid dispersions, the disperse phase of a foam is generally not
long-lasting. One reason for this transient nature of bubbles is that they are
generally more deformable than emulsion droplets (or certainly solid particles)
and this makes them more susceptible to coalescence. For instance, the gas
bubbles in many foamed products are large enough to be more easily deformed
by the hydrodynamic forces operating during processing. Coalescence can also
be considerably accelerated by a rapid drop in pressure, for example, in the exit
of foam from a mixing or aeration chamber, or in the extrusion of an aerated
product from a nozzle, during dispensing, filling, etc. This pressure change can
lead to significant bubble-bubble coalescence, or to coalescence of bubbles with
surface of the product.
Up until now there have been relatively few studies 1-5 that have investigated
this problem, either experimentally or theoretically. We reported previously 2 a
technique that allows one to observe and measure the behaviour of air bubbles
undergoing expansion (or compression and then expansion) due to changes in
pressure of the order of a factor of 5 (which seems to be the typical range in
many processes). It was shown that such expansions can result in significant
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