Table 2.10. (continued) Lipid
Polyglycerol esters
Propyleneglycol esters Sorbitane esters
Polysorbates
Sucrose esters
Calcium and sodium stearoyl lactylates Ethoxylated ricinic oil
Examples and US FDA and EEC E numbers Citric acid esters of
monoglycerides (172.832) (E 472) (172.854) (E 475)
(17.854) (E 477)
Sorbitane stearate (solid) and sorbitane oleate (liquid) (172.842)
(E 491)
Polysorbate 80 (oleate-liquid) (172.480)
(E 433) (172.859) (E 173) (172.844) (172.846) (E 482) (E 481) ( - ) ( - )
Properties
A hydrophobic emulsifier (HLB typically less than 4)
A lipophilic emulsifier (HLB about 4). Forms an L2 phase with water
A hydrophilic emulsifier (HLB typically about 12-16).
Forms micellar solutions An intermediate emulsifier
(HLB estimated as about 8)
Emulsifiers with somewhat intermediate hydrophilicity (HLB about 6-8).
Dispersible in water
Uses
Emulsion products
Limited use due to restrictive legislation. Some use in the chocolate industry in combination with lecithin as a viscosity regulator Cake mixes
Emulsions
Frozen desserts, dressings, etc.
Limited use due to restrictive legislation and high cost level
Bread, coffee whiteners
Only permitted in the feed industry. Used as an emulsifier in self-emulsifying feed mixtures
the hydrophobicity of parts of the molecule itself. Mod-ified cellulose also gives a reduction in the interfacial tension.
A second interfacially active component may also induce the surface activity of weakly surface-active pro-teins due to strong intermolecular interactions. Electro-static interactions from anionic bile salts enhance the adsorption of cationic chitosan on emulsion droplets emulsified by using a mixture of phospholipids, choles-terol and such bile salts. Carrageenan interacts strongly with milk proteins, which is of importance in relation to the association to emulsions and to its application in stabilizing neutral dairy products.
Table 2.11. Characteristic properties of a range of important technical proteins Preparation
Dairy proteins Spray-dried
skimmed-milk powder Drum-dried milk
powder Sodium caseinate Whey protein
Egg proteins Egg white
powder Egg yolk
Animal protein Gelatine
Meat powder
Albumin Haemoglobin Fish protein
Vegetable protein Soybean protein
Potato protein
Gluten
Main components
Micellar casein, whey proteins, lactose
Micellar casein, whey proteins, lactose, fat
Free casein components Lactoglobulin, lactoalbumin
and lactose, depending on the processing; denaturates at 8O0C
Ovoalbumine, lysozyme;
denaturates at 75°C Phosvitin, egg yolk lecithin
Partially hydrolysed collagen
Myosine
Mainly BSA
Mainly myosin
Denaturates at 92° C
Characteristic properties
Soluble; reacts chemically (develops aromas) when heated
Low solubility due to denaturation during processing
Highly soluble and very surface active
Soluble under acid conditions, but coagulates with heating
Soluble; denaturates at air/water interfaces; very heat sensitive Dispersible and heat sensitive;
Both the protein and the lipids contribute to the functionality
Cool gelling properties
Technical meat powder is a highly denaturated protein product with no solubility Good solubility; denaturates
with heating, at 65°C.
Dark colour; poor solubility Technical fish powder is a
highly denaturated protein product with no solubility
Solubility varies depending on processing
Technical potato protein is a highly denaturated protein product with absent solubility Technical gluten has poor
solubility but swells with water
Use
Milk-replacing systems, such as baby food, ice cream, etc.
Chocolate, meat products
Emulsions and foam products Emulsifying or foaming
additive
Foam stabilizer
Emulsifier (e.g. mayonnaise and dressings), baking products
Texture-provided in spread emulsions and other food systems; foaming additive in candy foams
Feed products, although the BSA problematic has caused limitations and restrictions Meat products
Feed products
Feed products; of particular importance in fish feed
As milk or meat replacer in emulsion or gel products;
feed products Feed products
Feed products and as an extender in certain gel products
astaxanthin (a colouring additive used in the salmon fish industry).
5.1 Wettability
The process of dissolving or dispersing a dry material in water can be divided into several steps: the wetting step, when water is surrounding and penetrating the particles, the sinking, the dispersing, when the particles
are being separated and evenly dispersed, and finally, the dissolving process, when the primary particles dissolve.
The wetting process of a porous material (for instance, a powder bed) can be described by the Wash-burn equation (23). This equation relates the capillary force and the flow resistance, approximating the powder as parallel capillaries with a certain radius r, as follows:
dh
aycos(0)r
dt hrj
where h is the penetrating height, t the time, O the contact angle, y the interfacial tension, r the capillary radius, and rj the viscosity.
The above equation shows that the penetration is fast if, e.g. the radius is large, the viscosity is low, the interfacial tension is high and the contact angle is low (well-wetted surfaces). The initial part of the wetting is expected to be very fast (dh/dt goes to infinity when h goes to zero).
In the Washburn treatment, all parameters are assumed to be constants. However in many situations this is not true. The viscosity will increase if dry material is dissolved in the rising liquid. If the penetration is slow (a small capillary radius which corresponds to a small particle in a powder bed) and if the area is large, the vis-cosity increase will reduce the penetration significantly.
It can easily be observed that powdered sucrose is more difficult to disperse in water than standard sucrose. Sev-eral hydrocolloids are extremely difficult to dissolve due to the viscous liquid gluing the lumps together. Agglom-eration, enhancing the wetting by a larger radius and reducing the dissolution rate, improves the situation.
Ethanol in the first dispersing water stage could also improve the situation due to its lower solubility to most hydrocolloids.
A second parameter that varies is the contact angle.
The latter increases with increasing wetting rate accord-ing to various experiments carried out with movaccord-ing surfaces penetrating through an air-water interface.
This effect is particularly important when lipid-covered surfaces are wetted, causing the wetting in such sys-tems to be a linear process with time, rather than the retarding process as predicted by the Washburn equation.
5.2 Cohesive and repulsive surface interactions
A range of interparticle interactions operates between particles dispersed in air, as with particles dispersed in water. The balance between these has its main impact on the handling properties of the particles and is consequently of significant technical interest.
van der Waals interactions are very short-range in order and as powder systems commonly consist of fairly large particles (typically 20-2000 um), their magnitudes are most commonly insignificant.
Gravity and friction are more important when the density difference is large and the particle sizes are large.
For compact large particles (like seeds), such factors
dominate, while for smaller particles (like flour or coffee powder) other interactions are of more importance.
Electrostatic interactions in air are very different compared to the electrostatic interactions in aqueous systems, as there is no ion solubility and no double layer is formed. The source of the surface charges is also different. In air, there are no possibilities for acid-base interactions or for ion disassociation.
Charging in air is caused by static electricity and is stable for non-conducting particles. This makes the interaction coulombic in nature, with a range compara-ble to the radius of the particle. The electrostatic inter-actions easily dominate when the particle size is large, the density is low, and there is a low water content (low conductivity). Typical examples could be during the drying of milk powder or the transport of coffee powder.
Water bridges are formed between hydrophilic parti-cles as a result of water condensing in the gaps between the powder particles. The driving force behind conden-sation depends on the sizes of such gaps. The smaller the gap, then the stronger is the driving force. Hence, water bridges are formed under almost all normal humidity conditions, although they will be smaller under dry con-ditions than under wet concon-ditions. The strength of an individual water bridge is directly proportional to the radii of the surfaces in contact. The cohesive strength due to water bridges dominates in finely dispersed sys-tems, with the number of contacts increasing as a func-tion of the third power of the reduced particle size. A very wide particle size distribution reduces the strength due to a high frequency of large-small-large parti-cle interaction events. This feature is commonly used when small particulates, such as silica particles, are used as anti-cohesive additives in powder systems. Water bridges in a system with soluble material, e.g. sucrose, could dissolve fractions of the material and then precip-itate it to form solid bridges that may result in severe caking.
5.3 The chemical composition of the surface
The chemical composition of the surface influences the interaction (hydrophilic or hydrophobic), and the wettability (soluble, hydrophilic or hydrophobic), as well as the chemical stability. The chemical composition of the powder is a result of its technical processing, or, if non-processed, its biological structure.
If a powder, or a dry matrix, is generated from a wet system, the surface composition depends strongly
on the speed of drying. Slow drying (oven drying, drum drying and roller drying, among other techniques) allows the phase with the lowest interfacial tension (against air) to cover the surface. As an example, fat has been shown to cover the surface obtained by roller drying of whole milk. Similarly, the internal surfaces of fish protein pellets are covered with fat. With fast drying (e.g. spray drying), the air/particle surface is formed very rapidly from the liquid/water interface without allowing the system to equilibrate. Consequently, the composition of the air/particle interface is very similar to the composition of the air/water interface before drying. Surface-active material present in the system before drying (for instance, protein in a skimmed-milk system) dominates the powder after drying, while insoluble components usually stay well encapsulated.
However, fat may leak out of the surface due to air/water contact before drying or because of sensitivity towards the mechanical treatment after the immediate drying.
Freeze-drying is a special drying form carried out under solid/solid conditions. Ideally, this method should give a composition similar to the composition in solution, but if the ice contains non-frozen water the latter may cause enrichments at the ice/water interfaces that could appear in the final freeze-dried powder.
5.4 Characterisation of powder surfaces
A range of methods are used to characterize pow-der surfaces. Important techniques include methods for describing the cohesive character, compressibility, parti-cle sizes, porosity, density and wettability. However, in this section we will I focus on chemical characterization as a novel contribution to the understanding of powder properties.
The classical chemical surface analysis method is free-fat extraction. This technique gives a measure of the fraction of the fat component which is accessible to the solvent used during a rapid extraction. The data obtained are only a crude measure, which is very sensitive to the amount of exposed surface.
A new technique has been developed using Electron Spectroscopy for Chemical Analysis (ESCA) (24). This method is based on the surface sensitivity of ESCA, where the analytical depth is about 2 nm, which allows the outer surface to be analysed separately from the bulk of the material. ESCA gives the elemental composition (except hydrogen) which thus allows the user to divide the surface into components of the same number as the number of elements presents (normally three, i.e.
protein, carbohydrate and fat).