Chemistry Reference
In-Depth Information
moving the included phase volume back towards the full-fat version.
Not surprisingly, the low-fat alternative does not provide the same level
of lubrication as the full-fat original.
Clearly, this approach has not resulted in the air totally replacing
the fat droplets in terms of lubrication and probably not in terms of
consumer perception either. However, the air cells whipped into the
dairy cream alternatives were much larger than the oil droplets that they
replaced (at least an order of magnitude larger). So the question is what
would have happened if the air cells were in the same size range as the
oil droplets? The work required to make air cells so small is not trivial,
and then stabilising them against ripening needs a whole new science
area. This is the science area of air-filled emulsions, which has recently
been established. The initial work reported in 2009 (Tchuenbou-Magaia
et al
., 2009) used a novel group of proteins (hydrophobins), which
are found ubiquitously across all genera of filamentous fungi. Because
of their molecular make up and tertiary structure, they present some
fascinating physical properties. For instance, these proteins assemble
at air-water interfaces, and once there, they rearrange and aggregate to
give what is essentially a protein skin (Kisko
et al
., 2009). The gel-like
network seems to give elasticity to the interface (de Vocht, 2001), and
the mechanism of action is as if this elastic interface imparts a restoring
force, so as air tries to move via Ostwald ripening, the protein resists
this movement and keeps the air droplet size originally made in the
process. Hydrophobins will not only stabilise air-water interfaces, but
they behave in the same way at oil-water interfaces. The relevance of
this will be discussed shortly.
The approach reported so far involves the production of very small
air cells (less than 10 µm) using sonication. By doing this, the re-
searchers report that air-filled emulsion droplets can be constructed to
give physical properties similar to the oil-filled droplets they replace.
This approach obviously offers an exciting new route for reducing the
fat content of foods.
The initial work from A. Cox (Cox
et al
., 2007) was limited from
a microstructural design point of view as they produced large air cells
specifically targeted for use in stabilising ice creams. More recent work
reported by Tchuenbou-Magaia
et al
. (2009) has shown that air cells
can be produced of the same size as oil droplets in a traditional oil/water
emulsion, i.e. 10 µm, and which are stable for months using this type
of protein. This shows the remarkable nature of hydrophobins when we
consider the ripening forces acting on such small air cells. In addition, by
understanding the molecular and rheological behaviour of hydrophobins
at air/water interfaces, the P. Cox group has developed ways to use
alternative proteins to give the same interfacial rheological properties.
It has been shown that these alternatives can be used to impart the same
