Instant Emulsions - Food Colloids: Self-Assembly and Material Science - page 416

volume, emulsifier type and the nature of the aqueous environment (pH, ionic
strength).
Milk proteins are used in many food emulsions because they are seen as
acceptable and natural food ingredients. However, delivering the milk powder
in an oil slurry means that the proteins have to hydrate before they can provide
emulsification of the oil droplets. We have found that, while the preparation of
good emulsions is possible, buffer salts are required to reduce the pH so as to
provide microbiological stability in the water phase of the final product. The
associated disadvantage, however, is that the low pH can cause premature
precipitation of the caseinate proteins with subsequent destabilization of the
emulsion. The addition of electrolytes (sodium chloride and buffer salts) at the
levels required also tends to inhibit or destabilize structured emulsion forma-
tion, because of the rapid lowering of the pH on emulsification and increase of
the ionic strength of the aqueous phase. These two effects can 'precipitate' the
protein before it has developed a stable structure within the emulsion. And
local 'flushes' of acidity on initial mixing can cause coating of caseinate
granules with precipitated protein before there has been significant dissolution.
But emulsions structured in the absence of acid may be post-acidified to well
below pH 4.9 without loss of stability. This means that post-acidification with
lactic acid or acetic acid is possible.
To overcome the problem of pre-acidification, stable emulsions may be
obtained in the presence of a suitable emulsifier such as lecithin. Being soluble
in the oil phase, but not to any significant extent in the aqueous phase, lecithin
can participate in the early stages of emulsification, as well as can act as a wetting
aid for powder dissolution from the oil phase. While some emulsions could be
formed in the absence of caseinate, they were found not to be stable to shear.
Adopting this approach now leads us to question the activity of caseinate as an
emulsifier. Many authors 10-14 have studied the competitive adsorption of ca-
seinate with phospholipid emulsifiers. Heertje 15 has suggested that lecithins are
not able to displace proteins completely from the interface in O/W emulsions,
even at high phospholipid concentrations. The concentration of caseinate used in
their study was 100 times lower than that used here. Additionally they were
concerned with displacing caseinate, whereas upon hydration of the caseinate in
this case, the protein might attempt to displace the lecithin. Here also we must
consider the dynamic nature of the process, where droplets are being created as
the caseinate is hydrating. Therefore, the competition is occurring at a virgin
interface, and the interfacially active caseinate is also aggregating as a result of
the pH decrease, which may kinetically trap the protein at the interface.
To test this hypothesis, lecithin was either fully or sequentially replaced by
Tween 20 (a polyoxyethylene sorbitan ester). This nonionic surfactant is much
more potent at displacing caseinate from the oil water interface. 16-19 Stable
emulsions could be created with Tween 20 at one quarter of the level of lecithin.
However, the emulsions formed were not as viscous. This provides evidence to
the fact that the caseinate is not interfacially active in the Tween system, and it
is the dual role of emulsifier and structuring agent that caseinate plays in
providing the thicker structures when mixed with lecithin. Post-acidification of