It is often said (at least by accountants) that the two most important ingredients in confectionery are air and water. Whilst this may appear a trivial comment, it is worth remembering that both play a key role in determining the texture of many products, so perhaps there is some truth in the comment after all!
Careful use of aeration can add considerably to the appeal of products, making hard products softer or crisper and giving the luxurious softness to products such as mallows.
In some products aeration is obvious and a key definer of the product (products such as “Crunchie”, “Aero”, Mallows, etc.) in other products its presence is less obvious but equally important (nougatines, foamed jellies).
There are a number of ways of aerating confectionery masses, including:-
– Chemical aeration by creating gas within the mass by a chemical reaction and then entrapping it by solidifying the mass
– Mechanical aeration where air (or another gas) under pressure is disperced into a mass containing a foaming agent by high shear agitation (mallows in particular)
– Aeration through boiling (most often for beverage granule production)
– Sugar “Pulling” where air is incorporated by repeated stretching and folding of a high boiled sugar mass
– Addition of a highly aerated mass to another component which may be difficult to aerate (for example adding a protein based foam to a nougatine or caramel mass)
In addition there is one unique product type (eg “Flake”) where a thin film of chocolate is folded in such a way as to incorporate air into the structure.
Whatever method is used there are certain points which must be considered carefully:-
– Subjecting many confectionery masses to high shear may result in fat release or premature crystallisation (although this latter is desirable in many pulled products to help shorten the texture)
– Because of the high temperature involved, chemical aeration is a potentially difficult process to control and in most cases produces significant and rapidly developing flavour changes
Chemical Aeration
Carbon dioxide gas can be generated quite readily by either heating sodium bicarbonate to above about 80°C or by reacting it with a mild acid such as citric acid. Chemical aeration is generally only used with simple sugar/glucose masses.
When the decomposition is achieved purely by heat the residual sodium carbonate and increased pH will cause rapid browning of a sugar/glucose mass with associated flavour development. If the mixture is kept at a neutral pH by adding acid to the mix then the browning reaction is more or less eliminated.
The browning reaction produces very desirable and characteristic flavours in products such as “Crunchie”, “Honeycomb”, etc. but this is much less desirable for mint or fruit flavoured products, hence the addition of acid to eliminate the flavour development reaction.
In a typical process, a slurry of finely ground sodium bicarbonate in glucose syrup is added to a high boil sugar/glucose mix at the cooking temperature. The mass foams and colours extremely rapidly with a large change in volume. The reaction is stopped by forming a sheet of the foam and cooling it rapidly either on a cooling table or for continuous production in a cooling tunnel.
Because of its high temperature and volatile nature this process is potentially quite hazardous and appropriate safety precautions and thorough operator training are essential.
Once cooled the sheet can be cut (typically using saws as it is extremely brittle) into individual units. As the now solid foam is very hygroscopic the units are typically coated with chocolate on line before packing in a moisture resistant film. Alternatively the solid mass can be granulated for incorporation into other products such as moulded bars but its hygroscopic nature makes storage very difficult
Mechanical Aeration
Many aerated products are created using mechanical shear and injection of (usually) air. For this to be successful it requires a foaming agent, a compatible recipe and a fairly low viscosity for the mass. Processes and equipment for both batch and continuous production are widely available
Foaming agents are generally protein based – (for example egg albumen, gelatine, soya proteins) but there is constant technical development and it is wisest to consult a specialist manufacturer who can offer advise based on the exact characteristics of the feed material and the product requirements.
Similarly there are a number of well established manufacturers of mechanical aeration systems who can offer advice on the best approach and equipment based on the product required.
In a typical process a sugar/glucose syrup is cooked to an appropriate temperature and a solution of a foaming agent is added. The batch is then whipped under pressure to the required density before the pressure is released and the product is deposited and cooled rapidly to stabilise the structure
It is generally very difficult to produce a foam when a material contains even a small amount of fat or oil as this will suppress the stability of the foam. Even oil based flavours and colours can give problems so careful formulation is needed during development.
The viscosity of the syrup needs to be fairly low so that the gas and air can mix easily to produce the desired bubble size. If the viscosity is too high it will be impossible to form a coherent, even bubble structure. Most mechanical aerators use at least some over pressure so that the mass expands when the pressure is reduced
The subsequent processes also need to be such that the aerated mass is not deaerated in subsequent steps, such as vigorous mixing or shearing in a depositor.
Although most often applied to sugar based masses, modern techniques allow chocolate be foamed more easily and deposited using relatively conventional techniques, producing a variety of interesting products in addition to the more traditional ones
One specialised form of mechanical aeration is the injection of gas (often carbon dioxide) under high pressure into a high solids sugar/glucose boil followed by cooling and solidification still under pressure. When subsequently granulated the gas in the bubbles retains it pressure so that when in the mouth the bubbles burst with an interesting sound and sensation. This is often refereed to as “space dust” or “popping candy” and is very popular in children’s products. It does need to be kept well packaged t prevent moisture softening the product and releasing the pressure
Aeration Through Boiling
A relatively low moisture sugar or composite mass can be aerated by the combined use of heat and vacuum. This does allow a wide variety of materials to be aerated, but generally in a fairly random manner due to the process involved.
If a high solids mass is heated to a temperature well above its boiling point under vacuum and then a vacuum is applied, much of the water present will vaporise. There is therefore a combination of the vaporised water forming bubbles in the mass and the consequent reduction in moisture of the mass immobilising it to a solid foam which often needs to be cooled before the vacuum is released.
To be successful this process requires a significant amount of work and understanding as the temperatures and pressures used, the starting and final moisture content and temperature are all important. Additional heat for the process is often supplied via heated platens in a vacuum oven (which may be batch or continuous) or even via microwave or infrared radiation.
This process is most frequently used to produce a large cake for subsequent granulation, for example in the production of some beverage drinks which could otherwise be very difficult to dry and where the aerated structure greatly aids dissolving and dispersion. Because of the way the foam is obtained this product has an “open cell” structure (the bubbles interconnect) and the water vapour in the bubbles is rapidly displaced by air when the vacuum is released
Much chocolate “crumb” is produced in this manner using batch or continuous vacuum ovens. Crumb feedstocks are typically both temperature sensitive and high in fat which makes other drying methods impractical yet need the aeration to make subsequent refining possible. Another requirement which makes this process more complex is the requirement for high levels of crystallinity in the finished product
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Addition of Aerated Masses
It is frequently attractive to aerate a product which could not be aerated conventionally, particularly products like nougatines, caramels and fudges where the viscosity and fat content preclude conventional aeration.
In this case the most common technique is to add a highly aerated, very stable foam to the product mass while it is still hot and fluid before rapidly cooling and forming the product.
This technique allows a much lighter and more easily chewed texture to be achieved than otherwise would be the case, but it does require careful operation as it is easy to destroy the aeration by excess temperature or aggressive handling.
Typically the aerated material will be a simple sugar boil with a foaming agent added before mechanical aeration – often under pressure – to a very low density. This will be added to an otherwise finished product such as a sugar/glucose syrup with colour and flavour or a caramel. Following this blending of the two major components liquid fat may be blended in to the mix (particularly for nougatines), although caramels and fudges do not generally need this approach.
It is important that the aerated mass portion retains its aeration but also remains reasonably fluid so that it can easily be combined with the main product and that the process for bringing the two components together avoids the danger of loosing the aeration through excessive temperature or shear
This technique is particularly suitable for children’s products where an extended chew is desirable but without aeration the product may be too tough to eat easily. In addition harder caramels and fudges can be given a better initial texture by mild aeration and can be made less satiating.
Sugar Pulling
Sugar pulling is one of the oldest techniques for changing the appearance and texture of sugar confectionery. In this process a high boil sugar/glucose mass is stretched and folded on itself to give a silky, glossy appearance and (generally) a shorter texture. This is caused by the incorporation of air into the mass (generally in long channels rather than bubbles) and frequently a degree of crystallisation due to the intense agitation as the mass cools and looses some moisture. This is another process where any significant amount of fat or oil can cause problems as the mass will break rather than stretching, however flavours and colours are generally added before pulling to give even distribution
Sugar pulling is now generally done on automated machines although hand work is still practiced for small amounts. It is worth noting that older pulling machines with inadequate guarding were extremely dangerous and they have a long history of serious accidents. You should carefully review the guarding and operating procedures for older machines.
Folded Products
Although not conventionally “aerated” folded products do none the less gain much of their uniqueness from the open structure which results from this unique manufacturing process.
Perhaps the best known products of this type are Cadbury Flake and Mars Galaxy Ripple, but there are others.
Typically a system of rolls produces a coherent, thin film of chocolate or similar material prepared as a paste on the final roller. A knife then scrapes off this film and as it leaves the roller it folds into a cavity producing a unit with a folded cross section incorporating voids.
This is a difficult and demanding process to operate effectively depending heavily not just of the properties of the material, which must be soft enough for the folds to adhere to one another but stiff enough not to collapse, but also the precise design of the machinery which is closely guarded by the few manufacturers of this type of product.
Loss of Aeration
The stability of a bubble in a fluid is a very complex subject and the pure physics involved do not apply in the kind of high solids mixed systems which occur in confectionery products and intermediates.
It is worth noting that bubbles of different sizes have different internal pressures
Pi?T/r
Where Pi is the internal pressure, T the surface tension of the fluid and r the radius of the bubble.
There are conditions where aerated masses will loose aeration fairly readily and these points may help to act as a “trouble shooting” guide.
If the viscosity of the liquid phase is too low, the viscous forces on the bubbles will be much lower than the buoyancy of the bubble and the bubbles will rise to the surface and/or easily come into contact with one another. Another effect of the liquid phase viscosity being too low is drainage or thinning of the bubble walls in a highly aerated mass. This can initiate bubble to bubble contact and coalescence
As the viscosity of an aerated mass will be heavily dependent on its temperature it is important to cool an aerated mass quickly. When adding an aerated mass to another component the second component should not be too high in temperature
Because the pressure in a bubble is inversely proportional to its diameter, bubbles will coalesce when they come into very close contact with each other (no two will ever be exactly the same size). This will cause small bubbles to join to form single larger bubbles. This can happen particularly when the pressure on an aerated mass is released and the bubbles grow, taking up more of the volume and therefore moving together.
Shearing an aerated mass can also cause bubble coalescence as this will generate pressure differences around the shear elements
If the viscosity of the liquid phase is too high it will be difficult to disperse the gas phase into it and at best only large, unstable, bubbles will form.
Because the surface tension of the liquid phase is important in determining the stable bubble diameter anything which affects this will affect the formation and stability of the foam. Some ingredients may contain agents to reduce surface tension used in their manufacturing process and these residuals can affect aeration of a finished product. The presence or absence of these may vary between manufacturers or sources or even crop times.
Conclusion
Aeration of confectionery products is an extremely important process and strongly influences the texture and even flavour release of a product. It can make a tough product chewy, a hard product interestingly brittle and can also significantly change the physical appearance, making product glossy and attractive. A low level of aeration can make a clear jelly opaque and at higher aeration levels produce the elastic luxury of mallows.
Because aeration has such a dramatic effect on product properties it is important that the level of aeration and other issues such as bubble size are closely controlled and there is a high level of expertise available from both equipment and ingredient manufacturers to help with this.