For the purposes of this article, biscuits will be referenced but most of the challenges will also relate to wafers, cakes and similar baked and cooked products.
We can define secondary processing as any applied process that changes the form or shape of biscuits post baking and before packaging.
Secondary processing can range between simple dusting or sprinkling through to chocolate coating and more complicated multi component product assembly.
Photo credit: Sollich
Challenges can be categorised as follows:
Biscuit surfaces must not be in excess of c. 30°C before creaming, chocolate coating or similar secondary processing. As post oven temperatures can be significantly higher than this, biscuits must be cooled to a suitable temperature before further processing. If forced cooling is not an option, a “lay aside” process - where biscuits are trayed or boxed off and stored for a period of time before being fed back onto the line for further processing - can help stabilise biscuit temperatures. It can also help continue and stabilise biscuit dough fat crystallisation, which, in turn, can speed overall product cooling and crystallisation in the first days of the distribution chain. It is critical that product boxes are kept airtight during any extended (>24hr) lay aside period in order to avoid moisture uptake and accelerated staling later in the product’s life.
A biscuit temperature in the region of 26°C is ideal for chocolate enrobing - much lower than this can have the effect of crystallising the chocolate too rapidly - “shock cooling” the tempered chocolate and creating unstable crystal types which may result in fat bloom. Biscuit temperatures up to c. 30°C will retard chocolate crystallisation in the cooling tunnel whilst temperatures in excess of this will active destroy tempered crystals and cause the rapid onset of bloom and other quality faults.
Depending on the cream fat used in a cream formulation, biscuit surface temperatures in excess of 30°C will begin to melt the cream during any depositing or sandwiching process, causing it to be too fluid to maintain its structure.
Both the moisture level and its distribution within the biscuit structure need to be understood and controlled. If the moisture level is too high, the shelf life of the finished product may be compromised as texture will be adversely affected, resulting in consumer complaints.
Moisture within the biscuit will normally equilibrate over time. A Digestive type biscuit, for example, may have a core moisture of >3% and an outer moisture of <1% producing an average moisture of approximately 2.5%. Over time this moisture differential will reduce, even if the overall moisture may remain fairly constant.
If the baking process is not optimal, too much post bake moisture movement within the biscuit may cause product cracking - “checking” - and actual breakage over the product’s shelf life. Barrier technologies can be employed to prevent moisture migration between the base biscuit and fillings/coatings. For low moisture products such as wafers, a moisture conditioning step is often employed to increase and equilibrate moisture within the structure prior to further processing.
Low moisture products which are inadequately conditioned or processed may absorb moisture over their shelf life which can be another cause of checking. Inadequately conditioned wafers may deform to such an extent in wafer sandwich products that a “crocodiling” effect is created, where the two wafers separate and move away from a centre filling over time.
Product surfaces should be regular in shape and free from loose particles such as biscuit crumb or dust. Any surface debris will make coating secondary processes more difficult as the coating will have reduced adherence to the surface.
Loose debris will also be ‘washed’ from the surface during any enrobing process, increasing the working rheology of the coating and making the process increasingly inefficient and costly. In the case of a chocolate enrober for example, any debris entering the enrober will gradually thicken the chocolate requiring the overall specification of the chocolate to be adjusted using extra fat or additional emulsifier. Filters may be present within such systems, but as these will often be 1mm or higher in aperture, they will be relatively ineffective as product debris is crushed and dispersed in the coating mass through the action of circulating pumps.
Surface debris can be minimised through careful consideration of the product’s design and control of the manufacturing process. Metal baking bands should be kept clean for example, and extraction systems employed in situations where large amounts of product dust are generated - such as in the vicinity of any wafer cutting.
Lay aside handling can cause increased product breakage and damage. When refeeding product onto the production line, it’s essential to remove crushed or broken biscuits.
If coating weights are to be consistent, biscuit dimensions must be consistent also. For depositing processes, irregular biscuits must be removed from the production line to prevent additional waste. Biscuits can be removed manually or by automatic monitoring and rejection methods.
As the cost of any base biscuit is typically much lower than that of any secondary processed one, it is essential defect biscuits are removed BEFORE the secondary process to as to minimise the cost of the generated waste.
Biscuit fats can migrate from the biscuit and into the coating over the course of the product’s shelf life. This can affect crystallisation and subsequent hardness, heat resistance and melting point. When incompatible fats need to be used due to the overall design of the product or a particular claim - particularly lauric fats such as coconut, other nut oils or milk fat - the shelf life of the final product may need to be significantly reduced to prevent unacceptable deterioration of the product being reported by the consumer.
Where the fat content of the biscuit is deliberately reduced to target a particular fat claim, the direction of fat migration can be reversed, causing the fat to migrate from the coating into the biscuit.
Dough fats can be plasticised in a process which pre-crystallises the fat before mixing. This improves biscuit structure and fat distribution, and ultimately improves texture and reduces fat migration into the coating. When selecting a dough fat blend, it is important that its compatibility with the coating fat is confirmed before use by conducting a eutectic study to identify how the coating solids are affected by increasing levels of migrating dough fat, and vice versa.
Fat migration from the biscuit into the coating will typically soften the coating to an unacceptable level over time. The coating will melt at lower than expected temperatures due to the eutectic effect on the coating from the dough fat and can make it difficult for the product to be unwrapped cleanly before consumption.
Fat migration from the coating to the biscuit can often result in the coating cracking and, in some extreme cases, the coating separating from the biscuit base. Barrier technologies can also be employed to prevent fat migration between the base biscuit and fillings/coatings.
The dew point (the temperature below which water droplets can form as condensation on a product surface) is directly related to the relative humidity of the air - at the same air temperature, as humidity decreases the dew point will also decrease, as it increases the dew point will also increase. For example, at a constant air temperature of 20°C, at a RH% of 40% the dew point temperature is 6.0°C; at a RH% of 60%, the dew point increases to 12.0°C.
If product is stored in areas of high humidity and moisture can reach the biscuits, moisture contents can increase and adversely affect texture and shelf life. This moisture can then move within the product during its shelf life, causing enrobed coating to crack and separate. In extreme cases of moisture pickup, microbial spoilage can begin.
In oven halls in particular, the environmental temperature can be much higher than the optimum for secondary processing. It’s important that the process hall features a functioning air handling system that vents hot air and is able to control an ideal environmental air temperature of between 15°C and 20°C in the secondary processing areas. Air temperatures of 30°C or more will be too warm for secondary processes such as chocolate to proceed due to the risk of shelf life issues such as fat bloom. As mentioned, high temperatures will also impair depositing processes if the deposited material warms and becomes too fluid (biscuit creaming processes for example). Most process hall environmental management systems will control both relative humidity and air temperature.
A well-defined, continuously reviewed and improved process control procedure should exist to control the biscuit and environmental variables. This should include routine measurement, logging and review by line management of the following key parameters:
Each individual processing line should have its own process risk assessment completed and process control procedures documented. During extreme environmental conditions, such a warm or hot weather operations, further controls and procedures may need to be applied. For example, changes to warehouse storage and distribution chain controls such as the use of cooled areas or vehicles may need to come into effect. Process control data should be kept and revisited when product complaints are received in order to ensure a comprehensive investigation can take place, lessons learnt and the overall business performance can be continuously improved.
Any secondary process will be compromised if the processing equipment is not of an adequate specification, reliability and capability. All processes must be well controlled and monitored during production to ensure processing is accurate and consistent. All monitoring instruments and devices should be calibrated regularly to an acceptable, recognised, standard. Secondary processed materials such as chocolate can be several times more expensive per tonne than the base biscuit, so accurate secondary processing weight control is critical if over weights and product giveaway are to be avoided.
Operators should be properly trained in the operation of the secondary process equipment and be able to perform corrective controls. They should be sufficiently experienced to be able to cope with manufacturing incidents. Standard operating procedures must exist and be regularly reviewed and re-briefed to operators. Equipment monitoring and preventative maintenance procedures should be in place and applied.
The secondary processing of biscuits can be value adding and assist with product differentiation, consumer product recognition and ultimately, preference. In order for secondary processes to be successful, the biscuit, environmental and operational conditions prior to processing must be fully understood and controlled.
Photos resources: Sollich