Actually, the fat crystals, if sufficiently abundant, also form a space-
filling network thatimmobilizes the drops. If the volume fraction of water is
quite high, as in several low-fat spreads, there may be not enough crystals
for Pickering stabilization and formation of a network. If so, stability can
often be achieved by ‘‘solidifying’’ the aqueous drops, whereby their
coalescence becomes impossible. The agent of choice is gelatin, a solution of
which is liquid at, say, 30 8 C, and quickly gels (solidifies) upon cooling.
Question 1
A small food company makes an O–W emulsion with 10%by volume of oil and 1%
of a well soluble protein; a powerful stirrer is used. The emulsion shows appreciable
creaming, about 20%of the oil in one day. This is considered undesirable, and it is
concluded that the oil droplets must be too large. Since more efficient emulsification
equipment is not available, 1%of a monoglyceride is added to the oil, on the
consideration that this ‘‘emulsifier’’ will produce smaller drops. However, it turns out
that the emulsion so made creams faster. What would be a likely explanation?
Answer
Interpreting the results by using Eq. (13.24) suggests that the droplets must have
been of the order of a fewmm in diameter to explain the creaming. This is a quite
reasonable value. The specific surface area (6j/d) must thus have been of the order of
0.2 m^2 per ml of emulsion. Assuming the plateau surface excess to be 3 mg?m^2 ,
there must have been more than sufficient protein to cover the droplets. But 10 mg of
monoglyceride per ml of oil is then equivalent to 5 mg per m^2 of oil surface; it will
thus adsorb and may to a considerable extent prevent protein adsorption. The
monoglycerides may indeed have produced smaller droplets, but their stability to
aggregation and subsequent coalescence may have been greatly diminished. This is
because monoglycerides hardly cause repulsion across an aqueous film and give a
much lower interfacial tension than a protein. The droplets would thus gradually
become larger, hence cream faster.
Question 2
A food company makes an O–W emulsion with 10%oil and 0.1%whey protein
isolate as the emulsifier. The average droplet sized 32 is determined at 1.8mm. After
standing for two days,d 32 has increased to 2.2mm, and it is concluded that the
emulsion is unstable to coalescence. However, during further storage, the droplet size
does not increase any more. What would be the explanation?