Lecithins viscosity

Two of the earliest edible applications of lecithin, viscosity reduction in chocolate and confectionery products, and emulsification/antispatter properties in margarine, still enjoy wide popularity and represent outlets for large volumes of lecithin products. In addition, other early uses such as in bakery goods, pasta, textiles, insecticides, and paints, among others, are still active today. 

As stated previously, moderately and highly acetylated lecithins exhibit heat-resistant properties that are very desirable to have in many release agent applications (173). A natural crude lecithin is subject to thermally induced reactions that are responsible for the darkening and formation of insolubles that occur after prolonged heating. There are several viscosity grades of heat-resistant lecithins available, and lecithin viscosity varies with temperature. Low-viscosity lecithins can be easily sprayed without dilution, or prepared as part of a spray release system. 

Chocolate (0.3—0.5% lecithin) lecithin is a wetting agent and emulsifier. It facihtates mixing, saves processiag time and power, saves cocoa butter, stabilizes viscosity, iacreases shelf life, counteracts moisture thickening, and aids release of molded goods (see Chocolate and cocoa). 

Lecithin (qv), a natural phosphoHpid possessing both hydrophilic and hydrophobic properties, is the most common emulsifier in the chocolate industry (5). The hydrophilic groups of the lecithin molecules attach themselves to the water, sugar, and cocoa soflds present in chocolate. The hydrophobic groups attach themselves to the cocoa butter and other fats such as milk fat. This reduces both the surface tension, between cocoa butter and the other materials present, and the viscosity. Less cocoa butter is then needed to adjust the final viscosity of the chocolate. 

Recently, an electrorheological effect, i.e., an increase in the viscosity and dynamic shear moduli of lecithin/n-decane solutions in the presence of small amounts of polar additives (water or glycerol) when an external electric field is applied to the system, has been observed [65]. 

The huge variety of emulsions used as food, medicinal, cosmetic, and other industrial products make these colloids important practical systems in which the surface monolayers exert considerable influence. We have already discussed the use of lecithin to control the viscosity and the texture of chocolate in Vignette IV in Chapter 4. 

The viscosity of the binder was adjusted by varying the ratio of polyester to monomer so as to achieve a castable composition (Fig. 122). Lecithin was used to reduce the viscosity. 

Fig. 123. Effect of blends of fine and coarse ammonium nitrate on viscosity of a propellant containing 64% by volume solids and 0.025% by weight lecithin. Binder vicosity was 20 cF (according to Dekker and Zimmerman [24]).

The phosphoric acid esters of diacyl glycerides, phospholipids, are important constituents of cellular membranes. Lecithins (phosphatidyl cholines) from egg white or soybeans are often added to foods as emulsifying agents or to modify flow characteristics and viscosity. Phospholipids have very low vapor pressures and decompose at elevated temperatures. The strategy for analysis involves preliminary isolation of the class, for example by TLC, followed by enzymatic hydrolysis, derivatization of the hydrolysis products, and then GC of the volatile derivatives. A number of phospholipases are known which are highly specific for particular positions on phospholipids. Phospholipase A2, usually isolated from snake venom, selectively hydrolyzes the 2-acyl ester linkage. The positions of attack for phospholipases A, C, and D are summarized on Figure 9.7 (24). Appropriate use of phospholipases followed by GC can thus be used to determine the composition of phospholipids. 

Fig. 21. Micellar weight versus dielectric constant e for natural lecithin. Lightscattering in single (o), (x) in mixed solvents ( ) diffusion-viscosity measurements (mean values). [Kolloid Z. Polym. 195, w 27 (1964)1...

Pluronic copolymers with molecular weight of 11,500 have been used as a viscosity modifier and form a clear gel at 20 to 25% w/w in water. A solution of Pluronic undergoes a thermal gelation process in which a mixture of the polymer and water forms a clear solution at low temperature and then becomes a gel when raised to room temperature (above 21°C). A blend of Pluronic and lecithin is sold as Organogel for topical gel formulations. 

Various lecithin-based MEs were also characterized by Hasse and Keipert [131]. The formulations were tested in terms of their physicochemical parameters (pH, refractive index, osmolality, viscosity, and surface tension) and physiological compatibility (HET-CAM and Draize test). In addition, in vitro and in vivo evaluations were performed. The tested MEs showed favorable physicochemical parameters and no ocular irritation as well as a prolonged pilocarpine release in vitro and in vivo. 

By any appropriate conventional viscosimeter, or by AOCS Bubble Time Method Tq 1A-64, assuming density to be unity. Fluid lecithin having a viscosity less than 7,500 centipoises may be considered a premium grade. Using Precision cone 73525, Penetrometer 73510 sample conditioned 24 hours at 25°C. 

Consistency. Lecithins are available in both fluid and plastic (solid) forms. Fluid lecithins generally follow Newtonian flow characteristics. The viscosity profile of lecithins is a complex function of acetone-insoluble content, moisture, mineral content, acid value, and the combined effects of assorted additives such as vegetable oils and surfactants. Generally, higher AI and/or moisture content yields higher viscosity, whereas an increased AV often decreases viscosity. Certain divalent minerals, such as calcium and others, can also adjust the viscosity level. 

Dry lecithin is highly viscous, and the viscosity increases drastically and then falls off as the moisture content increases. Comparative conditions used for drying lecithin in the two types of drying apparatus are given in Table 24 (59, 130). 

Fluidizing. Fluidizing additives such as soybean oil, fatty acids, or calcium chloride can be added to adjust the viscosity. The viscosity of dried crude lecithin can also be decreased by warming it to a maximum of 60°C. The dried crude lecithin product (unbleached or bleached) can also be used to prepare a variety of grades of lecithin by removing the oil to increase the phospholipid content, or by separating the oil-free lecithin into alcohol-soluble and alcohol-insoluble fractions. 

As a variety of methods are available for modifying the emulsifying properties of commercial lecithin, the potential for improved, tailor-made, functional products is unlimited. The main functional properties are emulsification, antispatter, instantizing/wetting/dispersing, release/parting, viscosity modification, and baking applications. 

Producing low-viscosity, fluid lecithins as wetting, dispersing, and release agents. 

Crystallization control. Lecithin can control crystallization in various food systems. In foods containing sugars or fats, the presence of as little as 0.5% lecithin can produce altered crystal sizes and stmctures that can have positive effects on product texture and viscosity. This is important in cookie fillings, butter-containing maple syrups, ice cream toppings, and similar products (7). 

Confections. There are three major specific properties for lecithin in confections emulsification (e.g., caramels), anti-stick/release properties, and viscosity modification (e.g., chocolate) (175). None of these properties stand alone. For example, emulsification in caramels will influence shelf life and texture. In chocolate, viscosity modification will alter production costs and texture of the finished product. 

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