Whipped cream, partial coalescence

Emulsion stability is required in many dairy applications, but not all. In products like whipped cream and ice cream, the emulsion must be stable in the liquid form but must partially coalesce readily upon foaming and the application of shear. The structure and physical properties of whipped cream and ice cream depend on the establishment of a fat-globule network. In cream whipped to maximum stability, partially coalesced fat covers the air interface. In ice cream, partially coalesced fat exists both in the serum phase and at the air interface also, there is more globular fat at the air interface with increasing fat destabilization. Partial coalescence occurs due to the collisions in a shear field of partially crystalline fat-emulsion droplets with sufficiently-weak steric stabilization (low level of surface adsoiption of amphiphilic material to the interface per unit area). To achieve optimal fat crystallinity, the process is very dependent on the composition of the triglycerides and the temperature. It is also possible to manipulate the adsorbed layer to reduce steric stabilization to an optimal level for emulsion stability and rapid partial coalescence upon the application of shear. This can be done either by addition of a small-molecule surfactant to a protein-stabilized emulsion or by a reduction of protein adsorption to a minimal level through selective homogenization. 

Other t) es of interactions such as coalescence (Gurming et al., 2004a), depletion forces, or steric interactions on close approach of droplets can be studied by these methods. Thus, it is now possible to selectively modify the interfacial structure and to study the implications of such changes in terms of droplet interactions. For liquid fat droplets controlled (partial) coalescence, followed by solidification of the fat, is used to control aggregation or network formation, as a means of generating texture in whipped foods and ice creams. Partial coalescence should be particularly sensitive to the structure of mixed interfaces and the methods described above can be adapted to study this phenomenon. 

Whereas true coalescence is of limited importance in the case of milk and dairy products, partial coalescence is of far greater importance, in particular in the preparation of products such as whipped cream, butter,... 

In the case of milk fat globules, partial coalescence can lead to the formation of irregularly-shaped granules (e.g., butter clumps), or the formation of a continuous network (e.g., whipped cream or ice cream). Walstra et al. (1999) reported that the following factors affect the rate of partial coalescence in milk ... 

Whipped cream is a dairy product that relies heavily on partial coalescence for the development of structure, as it is converted from a viscous liquid into a viscoelastic solid during the process of whipping. Figures 2 and 3 illustrate the build-up of the... 

Fig. 2. A schematic representation of the structure of whipped cream, showing the role of fat crystals within the emulsion droplets and partial coalescence of the emulsion in stabilizing the air bubbles and trapping the serum phase into a continuous three-dimensional network.

Fig. 3. The structure of whipped cream as determined by scanning electron microscopy. A. Overview showing the relative size arxf prevalerKe of air bubbles (a) and fat globules (/) bar = 30 pm. B. Internal structure of the air bubble, showing the layer of partially coalesced fat which has stabilized the bubbly bar = 5 pm. C. Details of the partially coalesced fat layer, showing the interaction of the irxfividual fat globules. Bar = 3 pm (Ref. 16).

In ice cream the fat content is lower, so there is not enough fat to cover the whole surface of the air bubbles (Figure 7.15b). The discrete and partially coalesced fat droplets are somewhat hydrophobic because they are partly coated with emulsifier. As a result they adsorb at the air bubble surface. They stabilize the air bubbles by forming a barrier between them. They also increase the matrix viscosity (since they are suspended solid particles), which strengthens the films of matrix between the bubbles and hinders coalescence. The extent to which the partially coalesced fat in ice cream exists as discrete clusters or an extended network (as in whipped cream) is an area of current research. 

Whipping (churning) cream at warm temperatures leads to complete, rather than partial, coalescence. This results in the formation of large fat particles and eventually produces a fat-continuous material. This is how butter is made, and is the reason why excess fat coalescence during ice cream manufacture is known as buttering . 

Even though emulsions generally need to be as stable as possible, there are several food produets where the opposite effeet is desired, so a shelf-stable emulsion ean be eontrol-lably destabilized when required. Sueh emulsions are the basis for produets such as whipped toppings and ice cream and generally depend for their effect on the processes of partial coalescence (168-170) and destabilization by whipping air bubbles into the mixture, at which time the interfacial layer of the emulsions may be mechanically broken and liquid oil spread aroimd the air-solution interface. 

Whipped cream structure is strongly dependent on partial coalescence, during which it evolves from a viscous fluid to a viscoelastic solid [82], In the interfacial structure of air bubbles in normal whipped cream, sparsely distributed fat crystals measuring 1 p,m essentially lie in the plane of the air/water interface. For adsorption of fat to occur, the cream must be stored at the correct temperature, which allows an ideal SFC to be reached [91]. In defective whipped cream, large crystals penetrate the air/water interface of most bubbles (Brooker, 1990). 

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