Spray drying has been used in the food industry for about 150 years and is responsible for creating some of the most essential ingredients and products in the food industry today—such as milk powder, instant coffee, and powdered flavors. Learn the key steps in the spray drying process, the top benefits of this process, and the variables you should know for creating the perfect powder.
Common Spray Dried Foods
We encounter spray dried ingredients and food products all the time. Whenever a liquid has been converted to a shelf-stable powder, there’s a good chance that spray drying was used. The most commonly spray dried foods include:
- milk powders
- dried eggs
- instant coffees
- instant teas
- dried fruit juices
- honey powders
- molasses powders
- powdered flavors
Why Spray Dry?
While there are several drying processes available for food, the spray drying process is distinguished by its unique equipment that allows for rapid drying with minimal heat exposure. In spray drying, a liquid is sprayed through an atomizer into a chamber that contains streams of hot air. The moisture quickly evaporates, leaving behind solid powder particles that fall to the bottom of the chamber.
Spray drying is ideal for heat-sensitive materials and whenever a free-flowing, uniform powder is required. Whereas other drying techniques typically produce flakes that then must be ground to size, spray dryers create a free-flowing powder with a narrow size distribution, making a subsequent grinding step unnecessary. Furthermore, spray drying is considered the method of choice for commercial-scale encapsulation applications and is used to encapsulate flavors, carotenoids, and lipids.
The top benefits of spray drying over other methods of drying are:
- Suitable for heat-sensitive foods
- Produces fairly uniform particle sizes
- Creates a free-flowing powder
- Effective at encapsulation
Key Steps in the Spray Drying Process
Spray drying is a unique method of drying that relies on atomization to create a uniform, free-flowing powder and allows heat exposure to be kept to a minimum. The spray drying process consists of the following steps:
- Preparation of the liquid or slurry
- Adding the liquid feed to the spray dryer
- Atomization of the liquid feed to create droplets
- Drying of the droplets in a heated air stream
- Collection of the dried particles
Understanding Atomization Technology
Atomization is the characterizing feature of the spray drying process and plays a critical role in determining the quality of the finished product. It involves generating a vast number of droplets from a liquid stream, thus greatly increasing its surface area and allowing faster drying rate. For example, 1 m3 of a liquid can form 2x1012 droplets with a surface area of 60,000 m2.
Atomization can be achieved through single-fluid nozzle, two-fluid nozzle, or rotary disc atomizers which produce droplet sizes from 10 to 500 µm (ideally 100 to 200 µm), depending on the feed viscosity and composition.
When the atomized droplets come in contact with the heated air currents entering the chamber (which can be co-current or counter-current to the atomized stream), a series of simultaneous heat and mass transfer processes takes place. Heat is transferred to the product to evaporate moisture, and mass is transferred as a vapor into the surrounding gas.
Understanding Drying Kinetics
The drying process can be described as having two phases: the constant-rate period and the falling-rate period. In the constant-rate period, moisture evaporates rapidly from a saturated surface via diffusion through the stationary air film at a rate sufficient to maintain saturation. In the falling-rate period and as moisture removal progresses, the solute dissolved in the liquid reaches a concentration beyond its saturation concentration to form a thin shell at the droplet surface.
Kinetically, this stage marks a transformation from low- to high-temperature drying. Following this, and depending on inlet temperature, feed viscosity, and atomization variables, the droplets may follow one of two principal pathways, creating either small-dense or large-hollow particles. Dried particles are recovered with separation devices such as cyclones and bag filters or are scrubbed for further collection followed by cooling and packaging.
Creating the Perfect Powder
A number of variables influence the characteristics of the finished powder, including feed properties (e.g., viscosity and solid content), type of atomizer used (e.g., nozzle or rotary disc), and airflow factors (e.g., flow rate and direction).
Optimization of these variables can be achieved through a good understanding of the spray drying process to produce particles free of imperfections (such as holes, agglomeration, or indentation) and with the desired properties (such as flowability, dissolution, and bulk density).
As a leader in spray dried nutrients, Watson is committed to educating the food industry on the importance of spray drying for creating nutrient premixes with the best shelf life, handling properties, and more! Click to access our complete guide on Spray Dried Nutrients.