Editing Spray drying (section)

==Micro-encapsulation==
Spray drying often is used as an [[Micro-encapsulation|encapsulation technique]] by the food and other industries. A substance to be encapsulated (the load) and an [[Amphiphile|amphipathic]] carrier (usually some sort of [[modified starch]]) are [[Homogenization (chemistry)|homogenized]] as a [[suspension (chemistry)|suspension]] in water (the slurry). The slurry is then fed into a spray drier, usually a tower heated to temperatures above the boiling point of [[water]].

As the slurry enters the tower, it is atomized. Partly because of the high [[surface tension]] of water and partly because of the [[Hydrophobe|hydrophobic]]/[[Hydrophile|hydrophilic]] interactions between the amphipathic carrier, the water, and the load, the atomized slurry forms [[micelle]]s. The small size of the drops (averaging 100 [[micrometre|micrometer]]s in diameter) results in a relatively large surface area which dries quickly. As the water dries, the carrier forms a hardened shell around the load.<ref>{{cite book|url=https://books.google.com/books?id=4JvVoRFe7AkC&pg=PA179|page=179|title=Bioseparation Engineering|author=Ajay Kumar|publisher=I. K. International |year=2009|isbn=978-93-8002-608-4}}</ref>

Load loss is usually a function of molecular weight. That is, lighter molecules tend to boil off in larger quantities at the processing temperatures. Loss is minimized industrially by spraying into taller towers. A larger volume of air has a lower average humidity as the process proceeds. By the [[osmosis]] principle, water will be encouraged by its difference in [[fugacity|fugacities]] in the vapor and liquid phases to leave the micelles and enter the air. Therefore, the same percentage of water can be dried out of the particles at lower temperatures if larger towers are used. Alternatively, the slurry can be sprayed into a partial vacuum. Since the boiling point of a solvent is the temperature at which the vapor pressure of the solvent is equal to the ambient pressure, reducing pressure in the tower has the effect of lowering the boiling point of the solvent.

The application of the spray drying encapsulation technique is to prepare "dehydrated" powders of substances which do not have any water to dehydrate. For example, instant [[drink mix]]es are spray dries of the various chemicals which make up the beverage. The technique was once used to remove water from food products. One example is the preparation of dehydrated milk. Because the milk was not being encapsulated and because spray drying causes [[thermal degradation]], milk dehydration and similar processes have been replaced by other dehydration techniques. Skim [[Powdered milk|milk powders]] are still widely produced using spray drying technology, typically at high solids concentration for maximum drying efficiency. Thermal degradation of products can be overcome by using lower operating temperatures and larger chamber sizes for increased residence times.<ref>Onwulata pp.389–430</ref>

Recent research is now suggesting that the use of spray-drying techniques may be an alternative method for crystallization of amorphous powders during the drying process since the temperature effects on the amorphous powders may be significant depending on drying residence times.<ref>Onwulata p.268</ref><ref>{{cite journal |doi=10.1080/07373930701536718 |title=Crystallization of Amorphous Components in Spray-Dried Powders |year=2007 |last1=Chiou |first1=D. |last2=Langrish |first2=T. A. G. |journal=Drying Technology |volume=25 |issue=9 |pages=1427–1435}}</ref>