Giving it the hardshell

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The science behind encapsulation is complex but there's a lot more to the process than just coating. John Dunn reports

As consumers become more health conscious, food manufacturers are increasingly looking for ways to incorporate ingredients that can deliver health benefits without compromising the taste or performance of their products. Microencapsulation looks set to be the tool of choice.

Microencapsulation is the process by which tiny packets of gas, liquid, or solid active ingredients are tucked into a shell or coating of a protective material. This shields and allows them to pass unscathed through processing, storage, cooking and even digestion, to deliver their magic properties. These microcapsules can range in size from one micron to a relatively gigantic few millimetres. But mostly they are under 200 microns.

So why microencapsulate? Essentially, raw ingredients are reactive chemicals. When they get together, especially when dissolved in water, they react and any heat present accelerates the process.

Microencapsulation prevents these chemical reactions and protects key ingredients from environmental conditions that could spoil their effectiveness. So microencapsulation can be used to protect ingredients from heat, low pH, oxidation, light, and flavour loss. It can mask undesirable flavours and prevent interaction with other ingredients. It can improve delivery of the required effect, increase stability and shelf-life, reduce production losses and so increase plant capacity, and cut transport costs.

In a report a couple of years ago, Frost & Sullivan suggested that demand for encapsulation was growing by 10%, driven by the need to fortify products with health ingredients and by consumer demand for novel products.

The leading players in the food microencapsulation business, according to Frost & Sullivan, include Balchem Corporation in the US; Aveka of the US; TasteTech in the UK; and Micap. Micap is listed on the UK AIM stock market, but its research and development is based in Germany and it has a production facility in Ireland. But microencapsulation is a specialised, small and rarefied community with each company claiming its own area of expertise and proprietary techniques.

One reason for this ultra niche-ness is that there is no universal encapsulation technique to suit all ingredients. Microencapsulation technology ranges from conventional spray drying, spray chilling and fluid-bed encapsulation to more specialised hot-melt extrusion and coacervation. And choosing the right process for your ingredient depends on many factors.

What is the ingredient to be encapsulated? Is it solid, liquid, water soluble or hydrophobic? How is the ingredient/flavour/colour to be released from the microcapsules when needed? By dissolving the capsule walls, by mechanical breakage, by melting, or by digestion? How big do the particles need to be? What are the quantities and costs in mind? What kind of release is required? Gradual, sudden, or tied to pH, temperature, or some other factor?

Spray drying - the simplest of all microencapsulation techniques - involves the atomisation of a liquid (the ingredient, plus a modified starch or gum, and water) into a hot air stream to produce a fine, dry, water soluble powder.

It can be used for flavourings, colours, fats and oils. A wide range of equipment is available. It also offers high throughput, wide choice of encapsulation materials, low process temperatures, and good stability.

Spray drying can also be used to microencapsulate ingredients into fine, dry, oil-soluble particles. It typically uses an oil-based core surrounded by a water-soluble carbohydrate, such as maltodextrin, starch, alginate, or cellulose derivative. An oil-in-water emulsion is formed which is then atomised in hot air and the soluble coating material on the particles hardens as the water evaporates.

Spray chilling is similar to spray drying, except that the soluble material forms the core and the insoluble material, such as waxes, fatty acids, and edible oils, forms the outer matrix. The coating solidifies in a cooling chamber. Release of the core occurs when the outer shell melts under heat.

Fluidised bed coating is used for encapsulating solid particles. The coating material is atomised in an air current and evaporates onto the solid particles. Multiple layers can be formed by this process.

Coacervation is a batch process generally used for coating liquids. Typically, gelatine is dissolved in water. Oil is added and the liquid mixed until tiny droplets are formed.

When the gelatine mixture cools, it forms a coacervate, or cluster of oil droplets within the gelatine which shrinks and gets rid of water as the mixture cools, forming a capsule around the oil. Heat and moisture dissolve the capsules and release the oils.

And there are other specialised techniques, such as centrifugal extrusion; spinning disc separation; hot-melt extrusion; and co-extrusion.

Co-extrusion creates fibres containing the active ingredient within high viscosity sugars and carbohydrates which are then chopped into microcylinders.

Leatherhead Food International in the UK has done a lot of work on complex coacervation using chemical crosslinking to stabilise the capsules at high temperature. Dr Pretima Titoria, who leads this work, says the enquiries that Leatherhead is receiving now show a growing interest in microencapsulation. The main reason, she suggests, is the need to incorporate health and wellbeing ingredients into products. "Most of the healthy ingredients, such as fish oils, are sensitive and unstable. So if you encapsulate them you can protect them from oxygen, light, and processing."

Janis Sinton runs TasteTech: a vegetarian microencapsulation company that does a lot of work for the baking and chewing gum industries. Sinton sees the main drivers behind the growth in microencapsulation as the need for 'clean labels' (no E-numbers), shelf-life extension, and lower cost in use.

TasteTech, which exports over 50% of its output, uses spray drying and lipid encapsulation to protect a wide range of ingredients. In particular it has developed a microencapsulation technique for sodium bicarbonate in chilled and frozen doughs, such as bake-at home pizzas.

"By encapsulating the bicarbonate in the baking powder you can prevent the moisture causing it to react prematurely," says Sinton. TasteTech also encapsulates the bakery mould inhibitor sorbic acid. "If you use sorbic acid in a loaf it destroys the yeast. But by encapsulating it in fat, the fat melts and releases it after the yeast has stopped working."

The microencapsulation of food ingredients is a huge market at the moment, says Michael Norris, chief executive of Micap. "A lot of added-value ingredients are quite delicate and in many cases expensive, such as omega-3 and probiotics. So by encapsulating them you get a protective benefit and an enhanced delivery benefit."

Micap has its own proprietary yeast encapsulation technology for volatile essential oils such as garlic, mustard, and onion flavourings. Norris explains: "Yeast is a cell. It is designed to protect its DNA and it protects it very well.

"By piggy-backing on that we can protect essential oils instead. They will stay inside the cell until the consumer eats the product." When the yeast is rehydrated in the mouth, the ingredient is released, says Norris.

But the problem is not so much encapsulating a sensitive ingredient as releasing it, he says. "It's easy to put something inside a protective shell. The skill is getting it out at the right time."

Professor Ian Norton, who heads up soft solid microstructural engineering at Birmingham University in the UK, agrees. "One of the big issues is getting the release profile right. A lot of attention has moved towards getting the right release profile. Simple, straight encapsulation is not where the action is any more. It's about targeted delivery, getting it to the right place."

Birmingham University is currently working with Nottingham University on a government-funded project that is working to encapsulate salt. "We want to get a higher salt perception from a lower salt content," says Norton. "People adapt to salt. If you put salt in your mouth and hold it there, you don't perceive the same saltiness as when you first inserted it. So we want to be able to 'pulse' the salt release to keep reinforcing the fact that you have salt in your mouth."

Norton is also looking at nanotechnology to reduce the size of oil droplets so that they become invisible in liquids. The aim is to add oil-soluble bioactive ingredients to drinks without turning them cloudy. "If you can get a fat particle down to below 40 nanometres then you have clarity," he says.

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