Chocolate and Chocolate Flavoured Coatings

Addition of chocolate to biscuits either as a covering or by means of moulding is a very important type of secondary processing and is growing in popularity. Despite the high price of chocolate, sales of chocolate biscuit products are very substantial in all but the hottest countries of the world. Chocolate and chocolate flavoured coating have been described in the section on Ingredients.

Handling of chocolate and coatings

Chocolate is stored either as liquid in silos at about 49°C or in solid form as blocks or small pieces.

Prior to use in an enrober or moulding plant the chocolate has to be melted. Melting kettles are normally heated with hot water and it is important that surface temperatures are maintained below 60°C otherwise there is a risk of flavour change (and damage to the lecithin in the chocolate) - a metallic flavour may be detectable if chocolate has been overheated. The advantage of using small pieces of chocolate compared with larger blocks is that they are easier to handle and they melt more rapidly.

Tempering of chocolate

The quality of chocolate whatever its colour and flavour is very greatly enhanced by its appearance and snap. The surface should be glossy and the texture hard and brittle. These properties are derived by very carefully controlling the fat crystallisation as the chocolate cools.

Cocoa butter, in common with other fats, crystallises in up to five different forms (allotropes). The commonest types of crystal are, in descending order of stability β (beta), β' (beta prime) and ᾁ (alpha). If the fat is cooled rapidly all three types will be present, but with time both ᾁ and β' types will change to the β allotrope. The change involves liberation of latent heat of crystallisation and a physical re-arrangement.

If the cooling is very carefully controlled the desired gloss and hardness can be achieved by the formation of only the β form of crystals. This cooling must be commenced before the chocolate is applied to the biscuit and is known as tempering.

The temper of a chocolate can be defined in terms of the number and type of seed crystals it contains in the fat phase. When tempering chocolate the aim is to seed it with stable β crystals so that on further cooling, crystallisation is more likely to proceed in the desired form of β crystals only. The level of seeding is important. If there are too few crystals present the chocolate after it is enrobed or moulded must be cooled very slowly to avoid supercooling with the associated presence of unwanted ᾁ or β' crystals. If there are too many crystals the liquid chocolate will be very viscous and difficult to handle in the enrober. When there are too few crystals the chocolate is said to be under tempered; when too many it is over-tempered.

Correctly tempered chocolate should have about 4-5% of crystals. The lower the temperature of the chocolate the more rapidly will more crystals form and cause increased viscosity.

If no crystals are present in liquid chocolate, on cooling crystals will suddenly be formed of all the different forms. In time the ᾁ and β' crystals will re-arrange to the β form and a mottled, whitish surface (known as fat bloom) will develop on the chocolate surface. By seeding the liquid chocolate (tempering) the rate and type of crystallisation is controlled and a hard smooth and shiny chocolate will be achieved. A mass cooled to the most stable crystal form shows the darkest colour and maximum shrinkage, which is an important aspect particularly for moulding of chocolate. Poorly tempered chocolate will not shrink enough to release from the moulds and will be soft or "cheesy" in texture.

Thus, the preparation of tempered chocolate must be viewed in relation to conditions while the chocolate is being applied and the subsequent cooling conditions.

The β crystals of the largest glycerides melt at about 34-35°C, the β' at 27-29°C and the ᾁ at 21-24°C. Thus, by very gradually cooling liquid chocolate to about 31°C will permit only β crystals to form. The problem is that cooling must be very slow and if during handling the temperature drops lower there will be a progressive increase in seed crystals resulting in increasing chocolate viscosity. Also the growth of crystals at 31°C is very slow. The normal method of chocolate tempering involves cooling in a scraped surface heat exchanger so that all types of crystals are formed at the cool surface, then by mixing with warmer chocolate all but the unwanted crystals are melted. With time in the enrober, the number of β crystals increases to an unacceptable level. To avoid this some of the chocolate is circulated away from the enrober and is reheated and re-tempered. The rate of re-melt and re-tempering is balanced so that the viscosity of the tempered mass is maintained as constant as possible. The retention time in an enrobing machine prior to re-melt is determined by the quantity of chocolate held, the rate of through-put of the tempering machine and the rate that chocolate is taken away on the coated product.

It is quite difficult to set up ideal conditions in the chocolate which will maintain temper and viscosity at the optimum levels and much trial and error is involved. It will be appreciated that the higher the temperature of the chocolate, without loosing temper, the lower will be the viscosity and the longer before too many crystals grow. The maximum working temperatures in the enrober are about 33°C for dark chocolate and 31°C for milk chocolate.

Chocolate flavoured coatings, made from non-cocoa butter fats, do not need to be tempered. To know the best temperature to use any particular coating it is wise to obtain advice from the supplier and furnished him with details of the chocolate handling plant available. It is possible to use the coating at up to 50°C (but the fluidity will only be changed up to the melting point of the fat which is probably not more than 40°C) but at these temperatures the amount of cooling required will be significantly more.

It is unusual to try to use coatings and real chocolate alternately in the same enrober because the two materials are not compatible. It is possible to use coating in an enrober with a little chocolate left in it but one can certainly not satisfactorily put chocolate into one with some remaining coating.

Enrobing biscuits

3ck-Enrober-diagram-thumbGeneral arrangement of an enrober

The coating of products with chocolate or coatings has led to the development of sophisticated machines known as enrobers. As the handling of chocolate for enrobing must be continuous with means of tempering the chocolate and maintaining it in correct condition, many enrobers have tempering devices built into them. In other cases the tempering machine is separate, but sited very nearby so that a minimum of temperature controlled pipe work is required for conveying the chocolate.

Enrobing is essentially a mechanical process but close attention must be given to temperature control of the chocolate. The circulation within the enrober must ensure that there are no dead spots where the chocolate can become heavily crystallised and affect the rest of the mass. As stated above, provision should be made to adjust the retention time in the enrober in relation to the rate that the chocolate is taken away on the product.

Despite the fact that viscosity of the chocolate is so important in respect of chocolate performance and pick up rate it is unusual for an in-line viscometer to be used. Sampling for viscosity is complicated by the unstable nature of tempered chocolate.

The importance of temperature control has perhaps been emphasised enough but one should also point out that the enrober room should be kept between 25-30°C and hoods over the enrobing station, with or without heaters, should be used to keep the air around the chocolate at the same temperature as the chocolate.

Enrobers have an open wire mesh conveyor up to 1.4 metres wide which carries the biscuits through a bath of chocolate circulated from below and through a curtain of chocolate poured from above. In the case of only half coating, the curtain is not used. Various devices are used to remove excess chocolate from the biscuit before it is transferred to another conveyor for cooling.

The biscuits to be coated should ideally be at a temperature of about 25°C. If they are too warm and they may destroy the temper of the chocolate place on them and if they are too cool this may cause chocolate viscosity problems which will affect the evenness of coating and may be the chocolate pick up weight. The biscuits should be fed into the enrober as close together as possible, without touching or overlapping, to achieve the highest throughput at the lowest possible enrober speed. Any loose biscuit or wafer crumb should be removed before placing the biscuits on the enrobing wire, but provision is usually supplied to filter out crumb which falls into the chocolate during its circulation to the tempering machine. Smooth surfaces coat better than rough and of particular problem are biscuits with loose bases such as those that have pulled out due to sticking on the baking band. Chocolate is quite thick and it does not flow well into these cavities. For certain "difficult to coat biscuits" like wafer pieces and also at high production speeds, it may be necessary to have two coating stations particularly for the bases. Such "pre-bottoming" machines are essentially separate enrobers with short contact type coolers to set the chocolate before passing through the main enrober.

A roller may be provided to depress biscuits into the bath of chocolate so that coating is not only on the base but also up the sides to a desired level. Care should be taken to ensure that the roller does not pick up chocolate otherwise it may soil the tops of subsequent biscuits.

Having coated the product with chocolate the excess must be removed so that only the desired pickup is achieved. Pickup weight control is very important as chocolate is an expensive ingredient.

Excess chocolate on the tops of biscuits is blown off with an air knife directed downwards and of even velocity across the width of the enrober. The air must be warm and is thus re-circulated within the enrober under the hood.

A vibrating or shaker device encourages excess chocolate to run off the coated biscuits and also evens out any ripples over the surface produced by the air curtain. If the shaker is merely a ratchet and gravity arrangement it may give about 300 vibrations per minute but if on a torsion bar up to 800 vibrations per minute can be obtained.

There may also be one or more "scraping" rollers either under or after the wire conveyor, which scavenge chocolate from the bases.

Finally there may be a de-tailing roller, as the biscuit passes to the cooler conveyor. This "licks" the drips at the back of the product reducing the possibility of a tail on the cooling conveyor. The de-tailing roller is of small diameter and revolves at high speed snatching the chocolate away. Chocolate tails on enrobed biscuits may be a problem in packaging as they can be hard, sharp protrusions which pierce the wrapping film.

Fully coated biscuits are usually transferred flat onto the cooling conveyor, but half-coated products may be inverted before cooling. Inversion is usually achieved by means of a roller which is a set of discs situated and driven, immediately after the enrober wire. The biscuits adhere to the discs, follow them round and are knocked off to fall inverted onto the cooling conveyor. Scraper fingers clean the discs and the whole may need gentle heating to prevent chocolate build up.

Garnishing and decorating

After enrobing it is possible to decorate the top of the biscuit with thin streams of chocolate which can be oscillated to give patterns.

Also, a sprinkler may be used to add nut pieces, rice crispies, etc. onto the surface of the chocolate.

Moulding biscuits in chocolate

By comparison with plant for enrobing biscuits, that required for moulding chocolate around biscuits is massive and a lot more complex.

The mould sets are in the form of metal or plastic trays which are removable from a continuous chain that passes through chocolate depositing stations, a cooler, a de-moulding point and a return circuit that allows warming of the moulds before being refilled.

The chocolate must be tempered but the exact procedures for filling the moulds with chocolate and biscuit vary somewhat. The simplest arrangement involves partial filling of the mould with chocolate, introducing the biscuit and then filling the rest of the mould and wiping off the excess chocolate.

It will be appreciated that the size and shape of the biscuit is very important as it must fit into the mould but not too loosely otherwise an excess of chocolate is needed to fill the mould. If the biscuit is too big it will not fit into the mould!

De-moulding of the cooled product is a critical stage. If the de-moulding is imperfect the automatic operation of the plant is impaired. Release from the moulds is achieved because well tempered chocolate contracts on cooling. This contraction is a little less than 2% under ideal conditions. Points to watch in order or achieve good mould release are

1. the moulds should be clean and free from oxidation, condensed moisture and scratches,

2. the moulds should be at chocolate temperature or slightly lower when filled. They should not be too warm or too cold otherwise the chocolate temper will be upset and this will reduce the degree of contraction on cooling. Moulds that need cleaning should be dug out with a soft non-scratching implement such as plastic and be washed in warm soapy water. Good rinsing and drying is essential before re-use.

Conditioning of biscuits and wafers before enrobing or moulding

Chocolate which has been set is very rigid and inflexible. It can easily be cracked on deflection. As the moisture content of biscuits changes so do their dimensions. It is, therefore, important that biscuits which are to be coated or cast in chocolate are stable in their dimensions before being coated and are protected from moisture pick up afterwards.

Biscuits which pass straight from the oven cooling conveyors to enrobers may have moisture differences between centre and outside that will equilibrate with time (this may cause "checking" as well as chocolate cracking). This within-piece equilibration is a particular problem on wafers.

Biscuits which is not completely enrobed, either deliberately or because there are gaps or pin holes in the coating, may pick up moisture and cause the chocolate to crack and fall off. Well covered, fully enrobed biscuit will be adequately protected from outside moisture by the chocolate. These moisture problems should be considered when packaging is being designed for chocolate products.

Products coated or moulded in chocolate flavoured coating (chocolate substitutes) are not so likely to display cracking for the above described reasons because the coating is usually more flexible that real chocolate.

Conditioning usually implies a deliberate change in the moisture content of the biscuit before coating. This can be done by exposing the biscuits or wafers in a humidity controlled atmosphere or simply by storing the product to allow internal moisture gradients to equilibrate. If the moisture is increased too much the biscuits will lose crispness and the flavour may change. Therefore an increase to a maximum of 4% moisture is recommended.

Cooling of chocolate

Cooling is a continuation of the crystallisation process started by tempering. It is as important that conditions here are right for the cooling as it is that the tempering is good otherwise some unwanted crystals may be formed.

There are two important principles to remember. Chocolate temperature should always be changed slowly either down or up and chocolate should be protected from moisture, especially that which will form by condensation. Thus, cooling should not be rushed nor should the surface temperatures be allowed to go below the dew point of the atmosphere.

The simplest coolers are convection systems where chilled air is circulated in such a way that the centre of the tunnel is at the minimum temperature. However, there is always the danger that the air at the mouth of the cooler will shock cool the chocolate and that humidities of the air within the tunnel will be high. It should be remembered that in addition to dew point condensation problems, dark chocolate will absorb moisture from atmospheres whose relative humidity is above 80% and milk chocolate from air at relative humidity of 75% or above. Air in cooling tunnels should be dried and re-circulated. Drying can be achieved by pre-cooling fresh air drawn in to much lower temperatures than that at which it will be used.

Air temperatures in subsequent tunnel zones of 16-18°C, 10-13°C and 16-18°C should suit most conditions. Minimum cooling time will probably be about 3.5-4 minutes for thin coverings of dark chocolate, 6-7 minutes or longer for milk but maybe as long as 25 minutes for heavier, thicker quantities and that in moulds. The presence of the butter fat in milk chocolate has a delaying effect on the crystallisation of cocoa butter.

Other means of heat transfer have been exploited for chocolate cooling. Radiation has proved very satisfactory and by using a thin cooling conveyor over cooled plates, some conduction or contact cooling is also possible. In radiation coolers heat radiated by the chocolate as it cools is absorbed on black cooled surfaces above and on the sides of the tunnel. There is no deliberate air movement around the chocolate, though some natural convection must occur. The black surfaces are cooled either by chilled convected air behind them or they form the surface of water cooled radiators similar to those used for domestic central heating. Fins on the black surfaces improve the ability to transfer the absorbed radiant energy.

In strong contrast cooling arrangements for chocolate flavoured coatings require a shock cooling at the beginning of the cooling tunnel to achieve best results.

The conveyor used to take the chocolate pieces through the cooling tunnel is usually plastic coated. A high gloss finish is essential if chocolate in contact with it is to have a good surface also. The use of patterned or embossed designs on the conveyor, which gives a pattern to the chocolate on the underside of the biscuit, is not so common now as it was in the past. The conveyor should be thin to allow as much heat dissipation as possible and also to allow it to pass over a sharp nose-piece at the point where the product is transferred off it. As it flexes on the nose-piece the chocolate easily peels off and any tails, drips or other spilled chocolate will also become detached and fall away leaving a clean band. Scrapers may also be used to ensure that the band stays as clean as possible.

The return run of the conveyor should be outside the cooling tunnel so that when it picks up more product for cooling the band is at room temperature and not excessively cold.

Sometimes an operator is stationed between the enrober and the mouth of the cooler to check that pieces of coated product are not touching before cooling. Separation at this stage can prevent considerable wastage later. The economics of using this operator should be balanced against lower throughput which would result if the biscuits are fed into the enrober with more spacing.

Handling and storage of chocolate biscuits

After cooling, chocolate biscuits are handled and packed in a similar way to other biscuits. It is necessary that the chocolate should not be touched with bare hands as traces of moisture from the skin will develop as clear finger prints on the chocolate. Operators should be provided with light nylon or cotton gloves which should be exchanged for clean at each shift change.

Chocolate products must be stored in cool, dry conditions for obvious reasons. If the chocolate melts in store it loses its temper and when it sets again will produce fat bloom. Unwrapped chocolate stored in cold conditions may develop a film of condensed moisture when brought into warmer conditions.