Spray drying is probably the oldest encapsulation technique and the most often used for preparation of dry, stable, food additives, particularly flavors. Spray-dried flavors have been available since the mid-1930s (6). The process is economical, flexible, and adaptable to commonly used, readily available processing equipment. Generally it is accomplished in three stages. A flavor, commonly an oil, although water-soluble flavors are also fixed using this technique, is mixed with an edible, food-grade polymeric material, such as gelatin, vegetable gum, modified starch, dextrin, or nongelling proteins often in the ratio of one part flavor to four parts fixatives (5). An emulsifier is added and the mixture is homogenized to produce an oil-in-water emulsion with a small micelle structure (7). The emulsion is atomized using one of several different methods, introducing an aerosol into a column of heated air in a drying chamber. The small droplets develop a spherical shape with the flavor oil gravitating to the center, or core and the aqueous-hydrocolloidal phase forming the outside coating. Rapid evaporation of water from the coating's surface maintains a temperature below 100°C (212°F) even in air columns of much higher temperatures. Fortunately the dwell time (exposure to heat in the chamber) is relatively short, usually only a few seconds (6).
Water-soluble flavors (or other food ingredients) are also fixed or entrapped in edible hydrocolloids but the particles do not have a clearly defined core and coating. Rather the spray-dried particle tends to be homogeneous.
Spray-dried particles are usually produced in particle sizes less than 100 fi, an optimum range for easy water solubility, or dispersability. Where necessary spray dried particles may readily be agglomerated or granulated for dry mixes where greater uniformity of particle size is desirable.
Before the introduction of spray drying, dry flavors were produced by plating out liquid flavors on dry substrates, typically sucrose, dextrose, starches, salt, or gelatin, depending principally on the composition of the mix being flavored. Although this crude method provided good flavor rendition and was obviously inexpensive (really nothing more than mixing), it provided no protection against loss of volatile components and oxygen-sensitive fractions were readily degraded (5).
With the advent of spray-dried flavors, oxygen degradation was significantly reduced as was loss of volatiles through evaporation. The colloidal, polymeric coatings provided at least some measurable protection. Some commercial products began to be referred to as locked-in flavors or protected. Even encapsulated began to be used to describe some spray-dried flavors in the early 1950s.
While spray drying offered a step forward in obtaining stability, the process created a problem in obtaining good flavor rendition (from wet to dry). Air, hot enough to instantly vaporize water, is going to drive off at least some low-boiling molecules, and large volumes of hot air will cause some instant oxidation particularly with sensitive terpene-rich citrus oils. Some answers to the problems associated with spray drying flavors came from the flavor chemist who began tailoring liquid flavors using less oxygen-sensitive molecules and fortifying liquids with excess volatiles to allow for rounding off during drying.
Spray drying remains the most common method for producing dry flavors. Some highly sophisticated flavor labs believe emphatically that they can accomplish anything with spray dehydration that can be done with more recently developed encapsulation techniques. However, the inadequacies of the process are generally recognized and it is the desire for superior dry flavors that have spurred the search for new encapsulation techniques and spread these concepts into many other food ingredient applications.
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