Spray Drying Working principle

The working principle of spray drying is to disperse the material to be dried into very fine mist-like particles (to increase the evaporation area of the water and accelerate the drying process) through mechanical action and contact with the hot air to remove most of the water in an instant so that the solid matter is dried into a powder.

Advantages of spray drying

1.Retain the taste, color and nutrition of food

Spray drying is often the last step in the manufacturing process. It is through continuous spraying, mixing and drying to change the substance from liquid to powder. Among the many food storage technologies, the spray drying method has its unique advantages. Because the temperature used by this technology is not very high, while removing microbial contamination, it can still effectively retain the taste, color and nutrition of food.

2.Change substance's size, shape, and density and improve quality

Spray drying machine is usually used to remove moisture from raw materials. In addition, it has many other uses, such as: changing the size, shape or density of the substance, it can help add other ingredients in the production process, and help produce the most stringent quality products.

3.Reduce food weight and make it easy to carry or transport

The production of fruit and vegetable powder by spray drying can retain the nutrients of fresh fruits and vegetables, and the volume has been reduced by 70%, which is easy to eat and transport.

4.Remove toxicity of some foods

Some plants require a certain temperature to remove toxicity. For these plants, the spray drying technology is ideal. Taking soybeans as an example, the concentration process requires a certain temperature to remove a substance called trypsin inhibitors(this substance will hinder digestion and decomposition of protein).

Freeze Drying Working principle

Freezing water-containing materials below freezing point, turning water into ice, and then turning ice into steam under high vacuum. The material can be frozen in the freezing device before being dried. However, it can also be frozen directly in the drying room by quickly drawing into a vacuum. The water vapor generated by sublimation is removed by a condenser. The heat of vaporization required in the sublimation process is generally supplied by thermal radiation.

Advantages of freeze drying

1.Keep substances' biological vitality

Freeze-drying is carried out at low temperatures, so it is particularly suitable for many heat-sensitive substances, such as proteins and microorganisms. These substances will not be denatured or lose biological vitality.

2.Small loss of volatile components

When drying at low temperature, the loss of some volatile components in the substance is very small, suitable for drying some chemical products, medicines and foods.

3.Keep the original characters

During the freeze-drying process, the growth of microorganisms and the action of enzymes cannot be performed, so the original characters can be maintained.

4.Maintain the volume and structure

Because it is dried in a frozen state, the volume is almost unchanged, the original structure is maintained, and concentration does not occur.

5.Long shelf life

Freeze drying can remove more than 95-99% of the water, so that the dried product can be stored for a long time without deterioration.

Which Should You Choose? 

Freeze drying is widely recognised in the biopharmaceutical industry as the preferred method for preserving a wide range of pharmaceutical formulations when stability in the liquid state is inadequate, storage requirements are too stringent, or a solid form of the product is desired. 

Whilst freeze drying is certainly the established drying process across a variety of materials, ranging in sensitivity levels, due to the costs, and sometimes volumes associated with the technique, research has also been undertaken to investigate alternative methods, such as spray drying.

Although relatively new to the industry, spray drying does come with some benefits such as capability to work with higher throughput (more continuous rather than batch), amounts in a scalable manner -making it a viable option to lyophilisation, but only in certain situations. 

The Drying Process

The principle of freeze drying involves the initial freezing of a product, usually in a controlled manner to manipulate the ice crystal structure, which thereafter is placed in a vacuum where sublimation (or primary drying) takes place in order to remove the unbound water. 

Next comes secondary drying to sublime the bound water, taking the material down to a user defined residual moisture level. Given the target at this stage is bound rather than unbound/free water more energy is required to drive the process by raising shelf temperatures to +20°C or above, coupled with a low atmospheric pressure, causing ice to turn directly in to water vapour (by passing the liquid phase). 

Subject to the nature of the sample being dried on a given lyophilisation cycle, the delicate balance between temperature and vacuum is vital to ensure that a successful batch with no impact on product efficacy, is produced post drying. To achieve this, certain products may need to undergo freeze drying conditions ranging from 12 hours to 5 days, based on their inherent sample properties. 

Spray drying is considered to be a more simple (and faster) process, involving the conversion of a liquid formulation into a dry powder in one single step. The solution is atomised into fine droplets which are quickly dried straight after in a large chamber using warm gas. The resulting dry particles are then collected with a cyclone.

Although faster, and less expensive than freeze drying, one of the significant downfalls tends to be due to the high processing temperatures/ shear forces it demands, these of course, are what many customers seek to avoid in the stringently controlled pharma/ bio-tech industries.

Use in the Industry

Both processes can be used for a vast range of applications. For example, freeze drying is typically sought out for the preservation of different cell types, fine chemicals/ laboratory reagents, injectable vaccines, as well as food industry & dairy products. Because it is typically performed with product directly filled in vials or other containers, this processing method is best suited for formulations that do not require further processing after drying; additionally, vials can be sealed in-situ of the lyophiliser, thus avoiding potential contamination when the cycle is complete.

Spray drying, on the other hand, is more commonly associated with bulk rather than vial based processing. However, it is a common misconception that spray drying is only a suitable process for food and robust bulk pharmaceuticals, where contemporary research suggests it may be a valid methodology for use with some complex products i.e. microencapsulated bacteria and nanoparticulates, for example. 

Efficiency, Quality and Cost

It is widely accepted the costs associated with spray drying are less than those of of freeze drying, which has made the technique one of interest for certain markets; and, being more open to larger throughput potentials, spray drying can be deemed a ‘continuous process’, unlike the batch format associated with freeze drying.

Nonetheless, it is important to consider that, although spray drying is usually less in ‘upfront cost’, this isn’t always the case for more complicated formulations. Products that require multiple coating layers will need to undergo multiple coating processes, which in turn can be very time consuming and cost prohibitive, making a simpler formulation that is freeze dried a more cost effective solution. 

Moreover, it is the robust quality of product where freeze drying’s strength lies. Accurate control of low processing temperatures minimises any risk of intrinsic products properties, such as collapse, eutectic melt, or glass transition temperatures being exceeded, making freeze dried products of the highest of quality. The shear stress biopharmaceuticals can be exposed to during spray drying, combined with the required high processing temperatures can destabalise compounds such as proteins and damage product properties, which, studies have shown, ultimately diminishes product quality.