Food-grade Microemulsions As Nano-scale Controlled Delivery Vehicles - Food and Industrial Bioproducts and Bioprocessing - page 146

Long-chain triglycerides (e.g., with 18 carbons) are difficult to solubilize given their high
molecular weight, which limits their flexibility at the interface and restricts film penetration
and formation (Gaonkar and Bagwe, 2002). These have been often substituted by medium-
chain triglycerides (e.g., tricaprylin), where the smaller fatty acid chain lengths permit
greater flexibility at the interface (Constantinides and Scalart, 1997; Fanun et al ., 2001 b).
Also, medium-chain triglycerides have been used to enhance absorption and bioavailability
of water-soluble compounds such as calcein and peptides (Constantinides et al ., 1995 , 1996 ;
Kamm et al ., 2000 ).

5.7.2 Aqueous phase
Water is an essential component of microemulsions. In o/w microemulsions, water acts as
the continuous phase. Water's immiscibility with hydrocarbon oils combined with its
ability to hydrogen bond with surfactants results in the formation of oil droplets when
it is present as the major component (although this depends on the properties of the
surfactant). Formation of microemulsions with small droplets usually requires the presence
of a water-soluble co-surfactant (e.g., short chain polyols such as propylene glycol), as
discussed later.

5.7.3 Surfactants
Surfactants are surface-active compounds that consist of a hydrophilic head group attached
to a hydrophobic tail (usually a long alkyl chain). They have a high affinity for water or oil
depending on the dominant moiety (Walz, 1998; McClements, 2004). When present in
sufficiently high concentrations, surfactants form a monolayer at the interface between the
oil and water, with the hydrophobic tails of the surfactant orientated towards the oil phase
and the hydrophilic head groups towards the aqueous phase. There are four categories of
surfactants available in the food industry: ionic, non-ionic, zwitterionic and cationic.

5.7.3.1 Ionic surfactants

Ionic surfactants have been used to formulate microemulsions. However, their toxic nature
at concentrations normally encountered in microemulsions limits their application in foods
(Alander and Warnheim, 1989; Flanagan et al ., 2006 ).

5.7.3.2 Non-ionic surfactants

Surfactants from this group are commonly used to formulate microemulsions, due to their
low toxicity, lack of irritability and capacity to easily form microemulsions (Yaghmur
et al ., 2004). Examples include sugar ester surfactants (e.g., sorbitan monooleate),
polyoxyethylene ether surfactants such as Brij 96 [used in the detergent industry (Gaonkar
and Bagwe, 2002; Garti, 2003)] and ethoxylated sorbitan esters (polysorbates) such as
Tween 60 (polyoxyethylene sorbitan monostearate) and Tween 80 (polyoxyethylene sorb-
itan monooleate) (Gaonkar and Bagwe, 2002; Garti, 2003; Flanagan et al ., 2006). Mixtures
of non-ionic surfactants are far more effective than the other class of surfactants for
microemulsion formation, as these potentially eliminate the need for a co-surfactant (Friberg
and Venable, 1983 ).