Microencapsulation by Spray Drying
Microencapsulation is a process in which tiny particles or droplets are surrounded by a coating to produce very small capsules, which have many useful properties. This is a specialized form of edible packaging of a food ingredient. Of course, the material used to form the protective packaging must be safe to eat.
In its simplest form, a microcapsule is a tiny sphere having a uniform wall and with a hollow inside to hold a droplet of the desired ingredient. The material inside is referred to as the “core” and the protective outside sphere as the “wall.” Generally, the particle sizes range between 1 and 200 pm.
Many flavors and extracts used in candies, soup powders, other foods, and cosmetics are liable to be subjected to oxidative deterioration and loss due to evaporation. For example, if cardamom oil is blended with tea to be included in a tea bag, the volatile oil will gradually be lost by evaporation. If the oil is protected by microencapsulation, this will not happen during a specified shelf life. When the bag is dipped in hot water at the time of consumption, only then will the volatile oil be released, giving the rich flavor of cardamom to the tea.
One of the most successful methods of microencapsulation is spray drying. The first step in microencapsulation is to make a colloidal suspension of the core material in an aqueous solution of the wall material. Two classic wall materials are maltodextrin and gum acacia. Both of these give emulsions with low viscosities capable of being pumped through the atomizer of the spray dryer. Of these two wall materials mentioned, gum acacia is more efficient; however, maltodextrin is cheaper. In fact, among the naturally occurring gums or hydrocolloids, gum acacia has the lowest viscosity in solution form. In the late twentieth century, some specialized starch-based gums were made available for use in spray drying with greater efficiency.
A typical mixture of various ingredients taken for homogenization could be as follows: water 70 L, gum acacia 22.5 kg, and oil (such as cardamom oil) 7.5 kg. The oil represents 25% of the mixture other than water, which will be evaporated. In the final spray-dried powder, oil will be less than 25% due to losses. Maltodextrin gives a much lower loading in the final powder, while a higher loading can be obtained by specially modified starch designed for spray drying. This mixture, after being stirred with a ladle, is passed through a pressure homogenizer where a stable colloid will be formed. Each colloid particle will consist of a droplet of core material surrounded by an aqueous solution of the wall material. When such a suspension is passed through the atomizer of the spray dryer, each colloidal particle will dry into a powder. As explained earlier, the powder will consist of a central core of volatile oil surrounded by a spherical dry wall formed by the evaporation of water during the passage through the hot zone of the spray dryer.
Lipophilic materials such as essential oils and oleoresins, when spray dried with modified starch or gum acacia, will become readily water-soluble. When viscous products like oleoresin are to be spray dried, for the stability of the colloid suspension, it may be necessary to add an emulsifier such as polysorbate, in addition to the hydrocolloid, to the slurry.
In some materials such as coffee extract, which is soluble in water, there is no colloid formation. Very small quantities of coffee extract, in the form of tiny droplets, when sprayed will dry into tiny particles. In this case, there is no wall or core to be discerned.