Crystallization of emulsifiers

At temperatures above 25°C, the presence of emulsifier results in only a slight reduction in interfacial tension compared to a pure oii-water interface (7 25 mN/m). When the temperature is decreased further, a significant drop in interfacial tension (7) is registered due to interfacial crystallization followed by crystallization of emulsifier in the bulk oil phase below 15°C. The increase in 7 observed at temperatures below 10°C is artificial being caused by a viscosity increase due to the crystal network which has formed. 

Emulsifiers were dissolved in sunflower oil, and protein in the water phase. With increasing amounts of saturated mono-digiycerides in the oil phase, increased interfacial activity was observed at low temperatures. At a concentration of 0.1%, which is usual in ice cream mix, the drop in interfacial tension starts just below room temperature (15°C). At this concentration no visible crystallization of emulsifier takes place in the oil phase. 

The same techniques developed for bulk fats can be equally employed to measure the crystallization of emulsified oils. In these cases, the experiments tend to be easier as the contraction of the fat on freezing does not lead to the formation of air spaces in the sample, but, on the other hand, because a smaller volume of material is undergoing a phase transition, the magnitude of the signal change, and thus the precision, is lower. 

Mishima, Application of polyamorphism in water to spontaneous crystallization of emulsified LiCl-H20 solution. J. Chem. Phys. 123, 154506 (2005). 

W Skoda, M van den Tempel. Crystallization of emulsified triglycerides. J CoUoid Interface Sci 18 568-584, 1963. 

Ice cream is simultaneously both an emulsion and a partially solidified foam, so it comprises three phases at once. The ice cream would be too solid to eat without the air, and too cold to eat without discomfort. The air helps impart a smooth, creamy consistency. The solid structure is held together with a network of globules of emulsified fat and small ice crystals (where small in this context means about 50 xm diameter). 

The two main assumptions underlying the derivation of Eq. (5) are (1) thermodynamic equilibrium and (2) conditions of constant temperature and pressure. These assumptions, especially assumption number 1, however, are often violated in food systems. Most foods are nonequilibrium systems. The complex nature of food systems (i.e., multicomponent and multiphase) lends itself readily to conditions of nonequilibrium. Many food systems, such as baked products, are not in equilibrium because they experience various physical, chemical, and microbiological changes over time. Other food products, such as butter (a water-in-oil emulsion) and mayonnaise (an oil-in-water emulsion), are produced as nonequilibrium systems, stabilized by the use of emulsifying agents. Some food products violate the assumption of equilibrium because they exhibit hysteresis (the final c/w value is dependent on the path taken, e.g., desorption or adsorption) or delayed crystallization (i.e., lactose crystallization in ice cream and powdered milk). In the case of hysteresis, the final c/w value should be independent of the path taken and should only be dependent on temperature, pressure, and composition (i.e.,... 

In addition to particle size, the degree of crystallinity and the modification of the lipid are of relevance for drug incorporation and release. Lipid crystallization and a change of the modification can be delayed with very small particles and in the presence of emulsifiers [20,21]. 

Figure 2 Fat crystallization of bulk fat and emulsified fat of ice cream mix with (+E) and without (-E) emulsifiers after cooling to 5°C measured by pNMR.

Crystallization of supercooled fat in topping powders may be studied by NMR afterreconstitution in heavy water. Below room temperature spontaneous fat crystallization takes place under isothermal conditions in the presence of effective emulsifier (PGMS) but not with ineffective emulsifiers or without emulsifiers (Figure 4). 

Figure 4 Crystallization of supercooled fat at 15°C measured by pNMR in the absence or presence of emulsifiers (PGMS or GMS).

Monoglycerides and mono-diglycerides have low HLB values and cannot form micelles. They build up a multi-layer at the surface, resulting in a constantly decreasing surface tension as their concentration increases. However, in systems with proteins such as fat-free ice cream mixes, these emulsifiers behave as if they have a CMC. A possible explanation for this observation is that the unbound emulsifier in the fat-free mix is in equilibrium with the protein-bound emulsifier. Above a certain concentration of emulsifier in the mix, any surplus of emulsifier will adhere to the protein in the water phase after the surface has been saturated. The unadsorbed emulsifier is seen as very small crystals less than 200 nm by electron microscopy analysis4. ... 

If the liquid crystalline phase is included in the diagram, the general features are those in Figure 7 (38). At this temperature (the PIT or HLB temperature) increasing amounts of emulsifier first give rise to an isotropic liquid (S) in a small concentration range (A-B), followed by a phase transition to a lamellar liquid crystal (N) in the concentration range C-D. 

The behavior of a series of polyoxyethylene alkyl ether nonionic surfactants is also illustrative. According to Figure 11 the dioxyethylene (A) compound does not form liquid crystals when combined with water. Its solutions with decane dissolve water only in proportion to the amount of emulsifier. The tetraoxyethylene dodecyl ether (B) forms a lamellar liquid crystalline phase and is not soluble in water but is completely miscible with the hydrocarbon. The octaoxyethylene compound (C) is soluble in both water and in hydrocarbon and gives rise to three different liquid crystals a middle phase, an isotropic liquid crystal, and a lamellar phase containing less water. If the hydrocarbon p-xylene is replaced by hexadecane (D), a surfactant phase (L) and a lamellar phase containing higher amounts of hydrocarbon are formed in combination with the tetraoxyethylene compound (B-D). 

Walstra, P., van Beresteyn, E.C.H. 1975a. Crystallization of milk fat in the emulsified state. Neth. Milk Dairy 1. 29, 35-65. 

Blaurock, A.E. 2000. Fundamental understanding of the crystallization of oils and fats. In Physical Properties of Fats, Oils and Emulsifiers (N. Widlak, ed.), AOCS Press, Champaign. 

Solvents are important ingredients of emulsifiable concentrates and of solution formulations. When the formulation is to be used on crops, it is critical that the solvent be nonphytotoxic. The solvent must have a high level of solvent power if an EC is being formulated. Because most toxicants are insoluble in water, the solvent must also be water insoluble. Otherwise, when the EC is added to water in the spray tank, the solvent will mix with the water and leave the toxicant behind as a crystalline precipitate. The carrying power of the solvent, i.e., the amount of pesticide it will hold in solution, is important in the storage stability of formulations. If near its saturation point at ordinary temperatures, it may exceed this at low temperature with the result that solvent and pesticide may separate, causing crystal formation and phase separation (Terriere, 1982). 

---