Foam is a dispersion made up of gas bubbles separated by thin liquid layers. Foam is a crucial factor in a variety of food products, including beverages like coffee, cappuccino, fruit juices, milkshakes, carbonated beverages, ice cream, whipped cream, wine, etc. The texture and mouthfeel of a food can be significantly improved by the presence of bubbles, despite the fact that they rarely provide any nutritional value. They contribute significantly to the fizziness and flavour of soft drinks and beers in beverages, as well as adding their visual appeal. To achieve a satisfying taste and flavour, for instance, a stable foam must be placed on top of beer or cappuccino. One of the primary benefits of foamed food is that consumers often consider them to be lighter in weight, lower in calories and maybe even capable of reducing obesity.
It is also known as a liquid-gas dispersion since the liquid works as the disperse phase, while the gas acts as the dispersion medium. A dispersion’s excess interfacial free energy has been claimed to prevent all of them from being thermodynamically stable. The system splits into two layers quickly as a result of the gas bubbles rising to the top, where they may deform into a polyhedral shape due to significant density difference between gas bubbles and the medium. The surface equilibrium disturbed due to rise in water/air contact can be prevented by incorporating the surfactant solution that provides a resorting force that strives to restore surface equilibrium balance.
In general, bubbles undergo several phases of instability during processing due to disproportionation, which is mostly brought on by a large bubble size variety. As a result of this process, the gas in the smaller bubbles, which have a greater Laplace pressure, diffuses or disperses into the bigger bubbles, which have a lower Laplace pressure. In the case of coalescence, this procedure takes place when the thin foam layers are ruptured, causing two little bubbles to combine and create a bigger bubble, lowering the internal pressure. Bubble collapse are also caused by the drainage due to gravity, which results in liquid flowing into the Plateau boundaries (the intersections of the three bubble films), induced by suction and cause bubble instability.
Mechanism of Foaming
The intramolecular interaction of components based on attraction and repulsion forces is one of the fundamental concepts of foaming. Topological fluctuations at the air-water interface result in bubble formation in the liquid phase. Small bubbles are created during the disintegration of a big bubble in this process and bubbles are also separated from the free surface or the purging nozzle. It takes a lot of energy to create bubbles since the process is not spontaneous. Along with these procedures, the generation of bubbles also entails the manufacturing of the bubbles and the rupture of the air-solvent interface, which necessitates a significant amount of mechanical force. Several researchers think that the addition of surface-active compounds can raise interfacial tension. The science of foaming includes both chemical interactions between surface-active molecules and physical forces between molecules within bubbles, with combined account for the stability and coalescence of bubbles.
Foam in the milk is caused by the presence of surfactant molecules. High Molecular Weight (HMW) surfactants such as proteins and Low Molecular Weight (LMW) surfactants such as monoglycerides and diglycerides, phospholipids and free fatty acids present in milk assist in stabilizing milk foam. Protein (HMW) molecules help to stabilize milk foam by forming an extremely viscoelastic coating at the interface through intermolecular interactions. Similarly, LMW surfactants prevent air bubbles from destabilizing foam through migrating to the thinnest part of the film for restoring the thickness and equilibrium surface tension of thinning films. The process of stabilization by LMW is known as the “Gibbs-Marangoni effect”.
This is because the displacement of proteins from the interface by LMW surfactants is dependent on the concentration of LMW surfactants. As the concentration of LMW surfactants rises, a progressive displacement takes place. Therefore, the foaming properties of the system containing both proteins and LMW surfactants are inferior to those containing only one of the two. It is generally known that LMW surfactants have a deleterious impact on milk’s foaming qualities, particularly when the system has higher free fatty acids and fat.
According to Kamath et al. (2008a), milk containing up to 1.0 equiv/mL of free fatty acids produced foam that was stable and smooth. However, when the amount of free fatty acids in the mixture increased to 1.5-5.0. equiv/mL, creamy foam started to develop during the steaming process and as soon as the steam frothing stopped, the foam changed to coarseness. As the free fatty acid concentration increased, the foam’s coarseness increased as well. Munchow et. al. (2015) observed that the impacts of various fat levels (0.5, 0.9, 1.5, 2.6 and 3.5%, w/w) on milk’s foaming characteristics revealed that foam produced from milk with a greater amount of fat was less stable and consistent than foam produced from milk with a lower amount of fat.
Emulsion for Improving Foam Stability
A hydrophilic or hydrophobic material with low surface tension is required for keeping a stable bubble. Hydrophilic components suck in water molecules, whereas hydrophobic components interact with the gas phase of the bubble. Surfactants enable the remaining hydrophobic components at the interface to interact with one another and with other hydrophobic moieties, demonstrating that the presence of a surface-active agent enhances the surface activity of the interface. Proteins and polysaccharides are the two major categories of biopolymers that are used to stabilize colloidal meals. Due to their high surface activity and ability to produce thick viscoelastic foams, solubilized proteins are quickly adsorbed at the air-water interface during the generation of aqueous foam. In contrast to coalescence and disproportionation, this causes the resulting bubbles to remain effectively stable.
The self-assembly approach has been used by many studies to describe the well-organized pattern development of surface-active components at the air-water interface. This formation produces densely packed hexagonal/honeycomb-like structures. Several other researchers have claimed that this patterning strategy may stabilize the emulsion’s microbubbles for a year by bringing them down to sizes below 1 μm with the use of surfactant. A well-known phenomenon for increasing the stability of emulsions and occasionally foams is the employing surfactant/stabilizer.
Numerous studies are being conducted to create innovative plant-based foam-forming and foam-stabilizing agents, since it is anticipated that modern lifestyles and consumer expectations would boost the requirement for these emulsifiers.
Emulsifiers: phospholipids, plant proteins, sterols, saponins, rhamnolipids, etc.
Thickeners: polysaccharides, beetroot pectin, citrus pectin, gum arabic, etc.
Gums: guar gum, Gellan gum and Plant-derived polysaccharides.
Surfactant: Tween 80, sucrose stearate, sodium dodecyl sulfate, cetyltrimethylammonium bromide, sodium oleate, benzalkonium chloride and lecithin.
All the above-mentioned stabilizers have been utilized for a variety of purposes for many years and are regarded as essential ingredients in the food, pharmaceutical and cosmetic industries for supplying high-quality products with foaming ability, consistency, shelf stability and consumer appeal.
Methodology used for preparing Stable Emulsion
Coffee solutions have been prepared by using 2g of instant coffee powder dissolved in 50 mL of Millipore water, along with different concentrations of surfactants (Tween 80: 0.2 g/L.). These solutions were stirred for 2 minutes with a magnetic stirrer at 750 rpm for proper mixing, followed by homogenization for 3 minutes at 18,000 rpm. The addition of surfactants lowered the surface tension of the coffee solution. According to the study, the foam was stable up to 20 minutes while using sucrose stearate and Quillaja saponin as surfactant. Similarly, the foam was stable up to 10 minutes while using tween 80 as surfactant. This might be due to the micelle aggregation, which has aided in the stabilization of foam at the air-water interface, caused by the adsorption of surface-active molecules found in surfactants.
Currently, consumers prefer beverages that foam quickly or consistently. There has recently been a rise in the use of ready-to-use powder that can be quickly reconstituted with water to make high-foaming drinks in the beverage sector. These foamed beverages appeal to customers more than non-aerated beverages do. For the production of instant foamy soluble beverages like coffee, tea, milkshakes or chocolate drinks, a variety of techniques have been utilized, such as spray drying, freeze drying, mixing, agitation, etc. The creation of such foamed beverages makes extensive use of creamers, foaming agents, stabilizers, gums and several other ingredients [67–73]. Creamers are frequently employed as a quick fix to maintain long-lasting foam in beverages. To improve the stability of drinks, emulsifiers, proteins or surface-active ingredients are added. Overall, this study has shown that the formation of self-assembly of surface-active components in coffee is strongly influenced by the surfactant used for the stabilization of foam structure. Among these surfactants, coffee containing sucrose stearate showed the highest bubble count (320±7 at 40 s), with a minimum coalescence rate (0.012±0.005 μm/s).
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