Food-grade Microemulsions As Nano-scale Controlled Delivery Vehicles - Food and Industrial Bioproducts and Bioprocessing - page 155

5.11.2 Emulsifiedmicroemulsions
Few formulations based on double emulsions (in which droplets of the dispersed phase
further contain smaller dispersed droplets) are on the market as these consist of large,
polydispersed droplets (10-100 μm) that are thermodynamically unstable. Controlling
the release of compounds incorporated in double emulsions is normally achieved by sta-
bilizing the interface of the inner emulsion, careful composition of the oil and aqueous
phases, and/or stabilization of the secondary interface (Davis et al ., 1985 ; Garti, 1997 ).
However, most double emulsions tend to release entrapped compounds in an uncontrolled
manner. A novel possibility for the stabilization of the inner emulsion is to use microe-
mulsions. Pilman and co-workers (1980) reported the use of L 2 microemulsions dispersed
within the inner phase of water-in-oil-in-water (w/o/w) duplex emulsions, and stabilized
in a sodium caseinate solution resulting in emulsified microemulsions. In principle, these
emulsified microemulsions may be used as controlled delivery system for nutraceuticals
and pharmaceuticals.
Lutz and co-workers (2007) formulated a w/o/w emulsified microemulsions based on
a two-step emulsification process whereby the w/o microemulsion was dispersed in a
continuous aqueous phase containing a steric stabilization amphiphile - graft copolymer
Pluronic F127. Water up to 15 wt-% in the inner microemulsion and 30 wt-% of microemulsion
in the double emulsion was entrapped, in comparison to only 5 wt-% obtained in previous
studies carried out by Yaghmur and co-workers (2005), which suggested the possibility of
enhanced solubilization of entrapped nutrients.
5.11.3 Protection against oxidation/light
Microemulsions are known to enhance the efficiency of antioxidants (Flanagan and Singh,
2006). Moberger and co-workers (1987) studied triglyceride oxidation in microemulsions
consisting of soybean oil, sunflower oil monoglycerides and water. Soybean oil was stable
against oxidation for an additional 50 days upon incorporation of 5% (w/w) ascorbic acid
into the water phase compared to similar microemulsions without added ascorbic acid.
Components solubilized within microemulsions are generally protected from degradation
as they are localized within dispersed domains that, being thermodynamically stable, do not
break down with time, thereby limiting exposure. For example, Garti and co-workers (2003)
found that lycopene, which is highly photosensitive, could be stabilized via solubilization in
dilutable U-type microemulsions.
5.11.4 Controlled release delivery systems
Controlled release and delivery involves delivering a drug/nutrient at a controlled (desirable)
rate within the body. Along with film/polymer coatings, microencapsulation is one of
the most common techniques used in the development of controlled-release matrices.
In microemulsions, the location of an incorporated bioactive compound, whether in the oil,
water or interface, will affect its release pattern (Leser et al ., 2006 ).
As controlled-release matrices, microemulsions offer several advantages over emulsions,
namely (Tadros, 1993 ; Garti and Aserin, 1996 ; Aungst, 2000; Lawrence and Rees, 2000 ;
Guo et al ., 2006 ; Kogan and Garti, 2006 ; Spernath and Aserin, 2006 ; Mehta et al ., 2007 ;
Gupta and Moulik , 2008):