Microencapsulation is crucial for manufacturers who want to incorporate notoriously testy omega-3 oils into foods, and spray drying is the most commonly used technique. Srini Subramanian, PhD, explains the important role the right matrices, emulsions and equipment can play
Food fortification with long-chain omega-3 oils is greatly anticipated, yet can be a challenging proposition for food processors because oxidative deterioration leads to off-flavour formation. These oils are also oxidatively sensitive to the ubiquitously present iron and copper in food.
A good initial quality with a clean sensory profile of omega-3 oil source is a key aspect in food fortification. The most common sources of long-chain omega-3 oils are fish oils (DHA and EPA) from various sources including menhaden and tuna eye socket, as well as algae sources (DHA).
The p-Anisidine Value (p-AV) is an indicator of lipid oxidation that measures secondary oxidation products, such as aldehydes. It is a marker of oxidative degradation that occurs during extraction, processing and refining of oils. Thus, lower p-AV means lower oxidative damage and better initial quality of oil. Another index of oxidation, the ?peroxide value?, can be reduced to near zero during deodorisation irrespective of the oxidative status of incoming oil.
Why encapsulate? The high degree of susceptibility of these omega-3 oils makes a case for microencapsulation in a matrix that makes them suitable for food applications. Typically, microencapsulation results in a free-flowing powder.
Besides stabilising these omega-3 oils, microencapsulation allows food processors to include it in various products. The reasons are varied: for convenience of addition; product compatibility (eg, powders with powders); to protect from secondary thermal processes like baking and extrusion; to prevent ingredient interactions (as, for example, DHA interacts negatively with certain artificial colours and flavours); and, above all, to extend shelf life of the fortified food.
Microencapsulation began in the late 1800s with spray drying; today a plethora of technologies exist. These technologies can be broadly classified into two categories: ?chemical? and ?mechanical?.
Some of the chemical methods used today include complex coacervation, polymer/polymer incompatibility and interfacial polymerisation. Besides spray drying, mechanical methods include spray chilling, fluidised bed drying, centrifugal extrusion, spinning disc separation and hot-melt extrusion.
Microcapsules are small particles, ranging from 3-800 microns in diameter, containing from 10-90 per cent of an active agent or core surrounded by a coating or shell. The particle characteristics for final applications that are to be considered include particle size and shape, bulk density, flow-ability/caking, dispersibility, moisture content, appearance and loading. It is also important that the powder is made from a stable emulsion and upon reconstitution the ingredient remains stable.
Spray dryingSpray drying is a commonly used technique not only for drying various food products (eg, dairy products such as milk) but also for specific microencapsulation of various food ingredients, such as flavourings, colours, fats and oils. More than 90 per cent of the flavours and omega-3 oils that are microencapsulated today are produced via the spray-drying technique.
Some of the advantages of spray-drying technology are the availability of equipment, high throughputs, cost-effectiveness, a wide choice of matrices, relatively low and effective process temperatures, and good stability of the finished product. The spray-drying equipment comes in large variations with great degree of customisation.
The first step in microencapsulation by spray drying is emulsification of the active ingredient in a suitable matrix. The most commonly used matrix materials are starches (emulsifying starches and hydrolysed starches), gums (primarily gum arabic/acacia), and proteins like gelatine, milk proteins and soy proteins.
Although proteins are not broadly used for microencapsulation by spray drying, they are the choice for niche applications such as omega-3 oils. Proteins have excellent emulsifying properties and provide relatively greater oxidative stability, besides allowing for high loads of lipophilic omega-3 oils in the powder. Proteins are considered relatively expensive, and, because of high viscosity, they limit the in-feed solids content. The marketplace (perception and consumer demand) and regulatory restrictions also dictate the matrix employed for a given application.
Depending on the choice of matrix, the primary carrier is hydrated, often with heating, prior to forming an emulsion with oil. Emulsions, in which oil is the discontinuous phase and the water-soluble fraction is the continuous phase, are made by mechanical means.
Use of various types of high-pressure homogenises ranging from 2,500-20,000psi is common with either single or multiple passes. The higher the pressure, the finer the oil droplet size. Multiple passes make the oil droplet/particle size distribution narrow or less wide. And as the size of the oil droplet is reduced, the surface area increases exponentially and hence increases susceptibility to oxidation.
On the other hand, reduction in oil droplet size improves encapsulation efficiency and thus minimises surface oil or easily extractable oil, the so-called free oil. Thus, a good balance of encapsulation efficiency and stability are to be obtained for a given oil-type and matrix chosen.
Among the many types of spray dryers used in the industry, the main variables are size, shape (conical, box or flat-bottomed), airflow pattern (eg, co- and counter-current, mixed flow), and type of atomisation. Atomisation is accomplished with a centrifugal disc or wheel, a single-fluid high-pressure nozzle, or a two-fluid nozzle.
Conventional and so-called single stage spray dryers (with co-current flow) are among the most widely used spray dryers. The temperature of the material being dried is normally much below the outlet air temperature. The powder particle characteristics are dependent on various factors including the chamber type, nozzle type, feed solids content and drying rate.
Briefly, the use of pressure nozzles and higher feed solids content ensures a powder of relatively higher bulk density and narrow particle size distribution. With centrifugal disc atomisation, the higher the speed, the finer the dried particle. Typically, the average particle size from a single stage dryer varies from 25-120 microns.
Multi-stage (or two-stage) dryers are used when you are looking for an average particle size larger than 100 microns — and, more importantly, a powder that has improved dissolution properties due to agglomeration. Multi-stage dryers incorporate a fluidised-bed drying operation (or a secondary belt dryer) into a spray dryer to obtain the desired characteristics. Other advantages in multi-stage dryers include an ability to dry temperature-sensitive and sticky or hygroscopic products because drying is completed at lower powder and exhaust air temperatures.
A modified spray drying or congealing process, widely used for omega-3 oils, involves spray congealing in the atomisation step, followed by drying to evaporate the liquid. Also termed modified spray ?chilling,? the formulation is typically gel forming or the matrix solidifies upon cooling (eg, dispersions made with gelatine).
The heated dispersion that is liquid is then sprayed in a chamber held at a temperature that is cool enough to solidify the droplets instantly. A secondary spray of a free-flowing agent, such as starch, is used to keep the solidified particles from agglomeration and prevent sticking to the wall. The product is dried to the final desirable moisture in a fluid bed dryer or a conveyor belt/filter-mat system at a temperature suitable to the active ingredient and below the melting point of the matrix. The dried microparticles are cleaned with either air or sieves to remove the excessive free-flowing agent.
Srini Subramanian, PhD, is principal scientist at Martek Biosciences in Colorado.
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