Microencapsulation of an ingredient can be done via multiple methods including spray drying and prilling with prilling being discussed here. Though the terminology defining prilling can be inconsistent (also referred to as spray cooling, spray chilling, and spray congealing), the micro-encapsulation methodology is more important.
Prilling in a nutshell is when a carrier entraps droplets of the liquid additives and is then solidified via crystallization (1). Okuro et al. define prilling as a “fat-based system, which involves the addition of the component of interest to a molten lipid carrier” which is then fed through an atomizer nozzle that comes in contact that with an environment “that is cooled below the melting point of the matrix material” resulting in a solidified material “due to the heat exchange between the molten material and cold air (2).” In other words, a water soluble ingredient is dispersed in a melted fat or wax and sprayed into a room temperature or cooled chamber where it solidifies. If the chamber is at room temperature, the melting point of the encapsulation material is between 45-122°C as opposed to 32-42°C if the chamber is cooled (3). The resulting prilled spherical particles have solubility based on the hydrophilic/ lipophilic nature of the carrier chosen.
Utilizing the spray chilling process is a growing trend in the food and supplement industry. Numerous benefits are provided including extending the shelf-life of an active by either protecting them against oxidation or by preventing reactions with other components that are as well present in the final application (3). Prilling also allows for spherical particles that not only flow better due to their spherical nature but also allow for the active ingredient to be uniformly distributed within the entire particle volume (2). Proper selection of the carrier can also result in the impacting the control of the release profile, taste and aroma masking, reduction of gastrointestinal irritation as well as increased viability in some applications (2, 3). Operational advantages to prilling include easy scale up, larger, controlled particle size (compared to spray drying), and free flowing powders produced not to mention reduced energy costs when compared to spray drying (2).
Though prilling allows for products to have a time released property due to the spherical particles being insoluble in water and the release of the active occurring only if the temperature is raised above the melting point of the fat or wax, there are some limitations to prilling. Okuro et al. discuss disadvantages such as the maximum amount of active that can be encapsulated is low hence spray drying is sometimes a more preferred method. Another disadvantage is that perhaps the expulsion of the active during storage which dependent on the processing conditions during prilling (2, 3). The proper choice of the active or encapsulated material is also key to prevent such expulsions as the degradation temperature of many food applications is low and therefore the active must be stable at the carrier melt temperature (2). Processing challenges during production are also another consideration since the active is charged via a melted carrier to the chamber, if the production lines are cooled, downtime can be significant. Another area that is often overlooked is that sometimes too fast a cooling rate can result in an unstable form of the particle which would then impact the final application in terms of texture and taste as well as may even result in particles floating to the top in an application. Finally, a significant limitation of prilling is that the end application has to allow for a lipid based coating as the prilling process results in water insoluble products as the carrier is a lipid or wax.
Application ranges for the prilling process are numerous as microencapsulation utilizes no/low heat and no organic or aqueous solvents to protect the active ingredient. Due to the aforementioned advantages, prilling can be used to extend shelf life for vitamins, minerals, flavors, probiotics, proteins, and peptides (2,3). Real world applications for prilled products include gums that give a burst of the active in a time controlled manner or baking goods with heat released ingredients. With the need for more shelf life products and controlled release of the active, prilling is being looked at more closely within the food and supplement industries.