B. The HLB System for Selecting Emulsifiers
Since the principal emulsifying agents are compounds containing both hydrophobic and hydrophilic groups, and since the phase in which the emulsifier is more soluble is generally the continuous phase, the type of emulsion produced (i.e. O/W or W/O) can be predicted on the basis of the relative hydrophilic-lipophilic properties of the emulsifier.
According to the hydrophilic-lipophilic balance (HLB) concept, each of the surface-active agents can be assigned a numerical value representing its hydrophilic-lipophilic balance.
Experimental determination of the HLB number for a given emulsifier is a tedious process.
However, this value may be calculated with satisfactory accuracy based on easily determined characteristics of the emulsifier.
The following equation was suggested by Griffin for polyhydric alcohol, fatty acid esters:
HLB = 20(1 - S/A)
S is the saponification number of the ester
A is the acid number of the acid.
In certain cases, where accurate determination of the saponification number is difficult.
HLB = (E+P)/5 is used
E is the weight percent of oxyethylene
P is the weight percent of polyhydric alcohol.
When ethylene oxide is the only hydrophilic group present the equation is reduced to HLB = E/5.
HLB numbers for some common emulsifiers are listed below.
The solubility of emulsifiers in water generally follows their HLB rank.
As a rule, emulsifiers with HLB values in the range 3-6 promote W/O emulsions; values between 8 and 18 promote O/W emulsions.
It has also been suggested that HLB values are algebraically additive so that the HLB of a blend of two or more emulsifiers can be obtained by simple calculation and that the blend of emulsifiers needed to produce maximum emulsion stability can be easily obtained.
This, however, is not always the case. Although the HLB concept is useful as a guide of comparing emulsing-forming or stabilizing properties, it suffers from a number of limitations.
First, commercial emulsifiers usually consist of a group of compounds rather than a single component.
This makes direct calculation based on chemical properties very difficult.
Furthermore, the HLB method does not take into consideration such factors as emulsifier concentration, mesomorphic behaviour, temperature, ionisation of the emulsifier, interaction with other compounds present, or properties and relative concentrations of the oil and aqueous phases.
Pure monoglycerides, for example, have an HLB value of approximately 3.8. Accordingly they would be expected to form only W/O emulsions. However, at emulsifier concentrations that permit the formation of protective mesomorphic layers around the fat globules, pure monoacylglycerols promote O/W emulsions.
Moreover, it is well known that O/W emulsions prepared from a blend of emulsifiers are usually more satble than those prepared from a single agent having the same HLB.
C. PIT as a Basis for Selecting Emulsifiers
It is obvious that temperature is an important factor in relation to the emulsion-forming characteristics of a surface-active agent.
An emulsifier that tends to be preferentially soluble in water at relatively low temperatures may become preferentially soluble in oil at higher temperatures at which hydrophobic interactions become stronger.
Determination of the temperature at which this inversion occurs provides a useful basis for emulsifier selection.
A strong positive correlation has been observed between the phase-inversion temperature (PIT) of emulsifiers and emulsion stability.
Toxicology - Based on extensive toxicological studies, including metabolic tests and long and short term feeding experiments with animals, Acceptable Daily Intake (ADI) values have been assigned to most food emulsifiers by the FAO-WHO Codex Alimentarius Committee, and some of these values are given in the last column of the table.
Specific Food Emulsifiers - A brief discription of the most commonly used emulsifiers follows:
1. Fatty acid monoesters of ethylene or propylene glycol are also widely used in baking.
A more hydrophilic ester can be prepared from a fatty acid and an alcohol, such as nonaethylene glycol.
2. Sorbitan fatty acids esters are usually mixed esters of fatty acids with sorbitol anhydride or sorbitan.
Sorbitol is dehydrated first to form hexitans and hexides, which are then esterified with fatty acids.
The resulting products are known commercially as "Spans".
These agents tend to promote W/O emulsions.
Compounds that are more hydrophilic can be produced by reacting sorbitan esters with ethylene oxide.
Polyoxyethylene chains add to the hydroxyl groups through ether linkages.
The resulting polyoxyethylene sorbitan fatty acid esters are commercially known as "Tweens".
In general, these compounds form hexagonal I liquid crystals in water and they can solubilise small quantities of triacylglycerols.
With larger amounts of triacylglycerols, transformation to a lamellar-type liquid crystal takes place.
The ability of an emulsifier to solubilise nonpolar lipids is important to the formation of phase equilibria at the emulsion interface.
3. Sodium stearoyl-2-lactylate (SSL), an ionic emulsifier, is a strongly hydrophilic surface-active agent capable of forming stable liquid crystalline phases between oil droplets and water, and thus can be used to promote very stable O/W emulsions.
It is obtained from the interaction of stearic acid, 2 molecules of lactic acid, and NaOH.
Due to their strong starch-complexing abilities, sodium (and calcium) stearoyl lactates are commonly used in the baking and starch industries.
4. Phospholipids such as soybean lecithin and those in egg yolk are natural emulsifiers that promote mainly O/W emulsions.
Egg yolk contains 10% phospholipid and is used to help form and stabilise emulsions in mayonnaise, salad dressing and cake.
Commercial soybean lecithin contains approximately equal amounts of phosphatidylcholine, phosphatidylethanolamine and inositol.
It is used to help form and stabilise emulsions in ice cream, cakes, candies and margarine.
Lecithin emulsifiers of different phospholipid composition and HLB characteristics can be obtained from commercial lecithin by fractionation based on solubility in alcohol.
5. Water-soluble gums, derived from a variety of plants, are effective in stabilising O/W emulsions.
They inhibit coalescence by increasing the viscosity of the continuous phase and/or by forming strong films around the oil droplets.
Materials in this class include gum arabic, tragacanth, agar, pectin, xanthan, methyl- and carboxymethylcellulose and carrageenan.
6. Gylcerol esters are a class of nonionic emulsifiers extensively used in the food industry.
Monoglycerides (monoacylglycerols) are prepared by direct reaction of glycerol with fatty acids or refined fats in the presence of an alkaline catalyst.
Commercial monoglycerides usually contain a mixture mono-, di- and triesters of fatty acids with a monoglyceride content of about 45%.
However, concentrated products containing more than 90% monoester can be prepared by mollecular distillation.
Distilled monoacylglycerols are commonly used in the manufacture of margarine, snack foods, low-caloric spreads, whipped frozen dessert and pasta products.
The hydrophilic nature of a monoester can be increased by increasing the number of free hydroxyl groups in the alcoholic moeity of the molecule.
Polyglycerol esters with a wide range of HLB values are thus produced by esterification of fatty acids with polyglycerols.
Polyglycerol chains containing up to 30 glycerol units can be prepared by polymerisation of glycerol.
The phase behaviour of monoacylglycerol-water systems is critical for optimum functionality of monoacylglycerols in aqueous systems.
With pure monoacylglycerols, the lamellar-type liquid crystal dominates for esters of the 12:0 and 16:0 fatty acids, hexagonal II or cubic type liquid crystals are usually produced from fatty acid esters with longer chains.
When the water content is low, unsaturated monoacylglycerols yield lamellar-type liquid crystals at room temperature.
By increasing the water content to approximately 20%, a viscous isotropic phase forms that transforms into a hexagonal II phase at temperatures above 70ºC.
If the water content is increased above 40% the viscous isotropic phase will separate as gelatinous lumps, making uniform distribution very difficult.
Commercially produced distilled monoacylglycerols are frequently used in the form of aqueous mixtures to facilitate their distribution in food products.
As pointed out earlier, the swelling capacity of distilled monoacylglycerols can be increased very significantly by neutralisation of the free fatty acids commonly present, or by the addition of trace amounts of ionic substances.
Dilute dispersions of the commercial products, when buffered to pH 7, gave clear homogenous dispersions that form a stable gel upon cooling.
Industrial products known as crystalline hydrates are prepared by heating a mixture of about 25% saturated distilled monoacylglycerols in water to about 65ºC, acidifying the resulting mesophase with acetic or propionic acid to pH 3 and cooling with a scraped-surface heat exchanger.
The product is a stable dispersion of tiny monoacylglycerol b crystals in water.
The so-called hydrates, which possess unusually smooth texture, are commonly used in the baking industry.
7. The hydrophobic character of monoacylglycerols can be enhanced by the addition of various organic acid radicals yielding esters of monoacylglycerols with hydroxycarboxylic acids.
Lactylated monoacylglycerols, for example, are prepared from glycerol, fatty acids and lactic acid.
Succinic and malic esters can be obtained in a similar fashion.
Acetylated tartaric acid monoacylglycerols are produced by reacting the monoacylglycerol with diacetyl tartaric acid anhydride.
The diacetyl tartaric acid esters, as well as the succinic acid esters, form lamellar liquid crystals that have limited swelling capacity in water.
However, as is true of distilled monoacylglycerols, their capacity to imbibe water can be increased drastically by the addition of NaOH.
Malic acid esters form cubic mesomorphic phases with water contents of up to 20% and hexagonal II phases at higher temperatures and water concentrations.
Succinic acid esters do not form mesomorphic phases with water, but they do exhibit mesomorphism.