Microscale bubbles could whip up a range of long-lived, stable foam products whilst giving formulations interesting sensory properties, according to research published in Science.
A collaboration between Harvard University and Unilever researchers found a way to create gas-liquid systems with tiny bubbles that remain stable for up to a year. This may help to significantly extend the life of foam-based food products, such as whipped cream, ice cream, sorbets, and mousses, according to Howard Stone from the School of Engineering and Applied Science at Harvard. The team was able to produce the stable bubbles by using a sucrose surfactant which forms a coating around the air bubbles. In addition, they noted that each bubble had tiny hexagonal patterns on its surface which, they explained, is caused by the amphiphilic nature of the surfactant used. Amphiphilic means the molecules have both hydrophilic (water-loving) and hydrophobic (water-hating) properties. The hydrophilic heads of the sucrose molecules in the surfactant sit on the outside of the bubble in contact with the water, whereas the hydrophobic chains of the molecule lie on the inside of the bubble. As the heads occupy more surface area than the tails this causes the surface of the bubble to bulge, resulting in the hexagonal patterns that can be observed. Dr. Rodney Bee, a retired Unilever physical chemist, initially produced the unusual bubble formation in the course of his research into finding ways to extend the life of foams and other gas-infused mixtures like ice cream. "We were interested in two things: Firstly, understanding how the size of the gas cells can influence the organoleptic and visual properties, and secondly, how to maintain their stability over time," Dr. Bee told FoodNavigator.com. Dr. Bee confirmed that Unilever maintain an interest in applying the technology but couldn't predict when a commercial product would hit the shelves. "We've established we can put this into the products that we want to put them in," he added. The paper reports the use of a sucrose surfactant which forms a coating around the air bubbles, but Bee added that others are being investigated. "The surfactant has to be able to form a crystalline layer that adsorbs to the surface," he said. "The interface between air and liquid is covered with surfactant molecules, both mono- and diesters, which are irreversibly pinned because of their low solubility in glucose syrup," wrote the researchers in Science. "The hydrophobic headgroups of the sucrose stearate sit in the aqueous phase, whereas the hydrophobic carbonyl chains lie inside the microbubbles. The observed bulging domains suggest that the surfactant molecules pack on the interface, with headgroups occupying substantially more surface area than the hydrophobic chains," they added. Problems with small bubbles Small micrometer sized bubbles rarely last longer than a few seconds and gas-liquid systems usually degrade with larger bubbles growing at the expense of smaller ones, explained the researchers. However, "we recognised that in whipped cream or ice cream small cells had been formed," said Dr. Bee. "We found that if you adsorb a surfactant and this formed a liquid-crystalline phase then the small cells would shrink, but this shrinkage would be arrested to find a kinetically stable state." These microbubbles not only influence the texture of the foam, but also how it looks. A major factor of the microbubbles is that they are very efficient at light scattering, said Dr. Bee. "The small bubble size gives the foam a rich white colour and a different kind of texture," Dr. Stone told FoodNavigator.com's sister site CosmeticsDesign.com. Unilever and food foams Researchers from the food, beverage and personal care company recently published other research into extending the stability of food foams. Lead researcher Andrew Cox from Unilever R&D, Colworth Science Park, is conducting research into the use of fungus proteins called hydrophobin HFBII to produce a foam with exceptional stability. "In the presence of a thickening agent to slow the rate of creaming, hydrophobin can stabilise foam to the extent where little, or no, air phase loss is observed, for over 4 months," wrote Cox and co-workers in the journal Food Hydrocolloids (doi:10.1016/j.foodhyd.2008.03.001). "In fact, we have an example where most of the air phase volume remains after 2.5 years storage at chill temperature." To read FoodNavigator.com's report on the hydrophobin research, click here . Source: Science 30 May 2008, Volume 320, Pages 1198-1201 "Interfacial polygonal nanopatterning of stable microbubbles"
Authors: E. Dressaire, R. Bee, D.C. Bell, A. Lips, H.A. Stone