Extruded Bread-chips - Possibilities and limits
In contrast to other direct expanded products, which are cut at the die-face, extruded bread chips are cut only after the expanded dough has cooled to some extent and has become firm.
This cut, made through an already expanded material, creates an open pored extrudate surface with an attractive optical appearance and texture, reminding one of the appearance of a cut bread slice.
While in the case of breakfast cereals and muesli components it is desirable to produce smaller forms, the target in the case of bread-chips are larger products. As the extrudate strands are cut, often in very thin slices, in the direction transverse to the direction of extrudate strand flow, the size of the product is dependant exclusively on the size of the cross section of the extruded strand.
There are however numerous physical and technologically limiting factors for the maximum cross section size of the expanded strand.
In order to be able to expand the overheated dough at the exit of the die into a well-formed strand, the dough in the die must be held under a definite pressure which is larger than the steam pressure.
Typical dough temperatures of 160-180 deg C demand hence a minimum pressure of 6 – 10 Bars. Higher the dough pressure at the die, more intensive is the expansion of the strand in the direction transverse to the direction of extrudate strand flow.
If one now selects a big die opening to achieve a strand with the largest cross section, it also causes simultaneously a lower die-pressure and one runs the risk, that the minimum pressure, below which the dough expansion occurs, is already reached even when the dough is inside the die. The result is a badly formed, frequently splitting strand, with a rough surface.
While in most cases the capacity of an extruder can be reduced to up to 40% of the rated capacity, in order to ensure that the production capacity can be matched with demand, during the production of bread-chips, a certain minimum throughput capacity, required to maintain the minimum necessary die-pressure in relation to a specific die, should not be crossed.
One can increase the die pressure through a higher viscosity of the dough (lower moisture) and thus cause a higher expansion in the direction transverse to the direction of strand flow. Dough with lower moisture hardens very quickly and is hence subjected to very little shrinkage after expansion, which is advantageous in the formation of strands with larger cross-sections. A lower moisture however also encourages the formation of very soft textures, which differentiate themselves significantly from the texture of "roasted bread”. Highly viscous, drier doughs however have lower elasticity, as a result of which significant tension is caused within the product even during the process of expansion (in statu expansii). These tensions lead to splitting of the strand surface and formation of irregular pores / texture.
In order to obtain a crunchy and uniform texture, which comes as close as possible to the texture of roasted bread, it is necessary to carry out the extrusion at sufficiently high moisture levels (17-20%). At this moisture level the dough remains soft and elastic for relatively longer time after expansion.
The softness of the dough demands, that the cutting machine is placed relatively far from the die, in order to ensure sufficient hardening of the strand before the cut is applied. The elastic characteristic of the dough however leads to a significantly stronger shrinkage of the strand due to the atmospheric pressure and hence to a final product strand of smaller cross section.
The use of full-grain raw-materials with corresponding content of evaporation nuclei in the form of size-reduced fibre material help to reduce the size of the bubbles and thus in forming more uniform textures. But they also reduce the expansion in the transverse direction.
Pins in the extruder die, which create corresponding spaces within the strand, lead to a higher die pressure and hence cause a stronger expansion of the strand in the cross section and reduce the internal tensions during expansion.
Due to the formation of the empty spaces within the product, the size of the form can be correspondingly enlarged. Development work is currently being undertaken at Schaaf Technologie to optimise the die system in such a way that at one end a bigger strand diameter can be achieved, without falling short of the minimum required die-pressure and on the other end the forces of tension, automatically generated during expansion within the strand cross section, can be minimised.
As per the current status of the technology it is possible to manufacture solid strands (i.e. with-out using pins or creating empty spaces within the strand) leading to a cut product with full surface.
Development trends for bread chips: In addition to the simple round bread chips, it should also be possible in the future to make more complex forms with open pores using this cutting technique.
The main objective would however continue to remain the maximisation of the product size, i.e. the surface size of the individual breads chips.