Biomedical Engineering Reference
In-Depth Information
Drying Technologies
Drying represents one of the most important costs in protein processing. Three techniques
most commonly used are freeze drying (lyophilization), spray drying or drum drying.
Freeze drying
Freeze drying involves the removal of water from frozen materials by sublimation under
high vacuum. The technique consists of three main stages: freezing, sublimation and
desorption. The sample to be dried (i.e., example wet protein precipitate), is first frozen at a
temperature below 0 °C, which should be sufficient to completely solidify the sample. The
frozen sample is then placed under vacuum with pressures lower than 610 Pa at a high
enough temperature to stimulate the sublimation of the free ice crystals. Water vapor is
mechanically removed from the environment by vacuum pumps or steam jet ejectors
(Mujumdar and Devahastin, 2000). To facilitate desorption of remaining water adsorbed by
the sample, the temperature of the sample is raised at the latter stages of freeze drying to
between 40 and 60 °C, which is sufficient to break any remaining bonds between the water
molecules and the sample.
Freeze drying is costly and requires considerable time and energy. However, it has several
advantages (Baker, 1997). It has a very low impact on the quality of the processed products
and enables the creation of a powder with a quick rehydration rate. Freeze drying may be
best suited for proteins targeted for the pharmaceutical and nutraceutical sectors, and
perhaps for high end food applications.
Spray drying
Spray drying is one of the most widely used industrial drying processes in the food industry.
It can transform low concentration emulsions, solutions or suspensions into powder and is
frequently used in the agri-food industry because it is quick and can be completed in a single
step. The product to be dried is pumped into the spray dryer and passes through an atomizer,
which sprays the suspension in tiny droplets. There are many atomizer models on the
market, each having specific advantages and disadvantages (e.g., vanes, nozzles, etc.). Hot
air fed into the drying chamber dries the atomized droplets. On contact with the air, the
water present in the droplets is quickly vaporized and the powder that forms drops out of the
drying chamber. More recent spray dryers offer the possibility of lowering the pressure
in the drying chamber, which allows products to be exposed to lower temperatures. This
minimizes the impact of temperature on heat-sensitive products, such as proteins, which can
denature at relatively low temperatures (80 °C) (Mujumdar and Devahastin, 2000).
Drum drying
Prior to the development of spray dryers, drum drying was one of the most commonly used
techniques for drying. Drum drying can be used to dry a variety of liquid materials and is
particularly effective for drying viscous liquids. Drum dryers consist of one or two horizon-
tally mounted hollow cylinder(s) made of high-grade cast iron or stainless steel, a supporting
frame, a product feeding system, a scraper and auxiliaries (Tang et al ., 2003 ). During
processing, steam at very high temperatures (up to 200 °C) heats up the interior of the drums
while the liquid material to be dried is applied uniformly as a thin layer onto the exterior
surface(s) of the drum(s). The heat evaporates the water from the material leaving a dry
product that is scraped off as the drum rotates towards the scrapers. The rate of drying and
final moisture content depend on the type of material being dried, residence time, feed rate,
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