The situation
A manufacturer of analogue pizza cheese had a product that melted well and did not stretch. On the pizza it flowed and covered correctly, and then cut cleanly with no pull at all, which the customer rejected on the first delivery.
Why it is hard
Melt and stretch are two different behaviours and they are routinely specified as one. Melt is flow: the fat softening and the matrix losing enough rigidity to spread. Stretch is long protein strands sliding past one another without breaking, which requires the strands to be both aligned and still connected to each other.
A system can have the first without the second, and increasing whatever produced the melt usually makes the stretch worse.
More flow means less of the connected structure that stretch depends on. In a casein-free analogue system the structure has to be built without casein. The structure has to be built from starch and added protein, and those two build it in different ways.
What was done
The two properties were specified separately with separate targets, which had not been done before. The starch was chosen for the cohesive, extensible character it brings rather than for viscosity, since that extensibility is doing part of the stretching. Protein type and level were adjusted to give strands long enough to slide, and the balance between the two was set by trial rather than by calculation, because the interaction is not additive.
What it shows
That specifying one of two required properties leaves the other to chance, and that the second one is usually what the customer judges.
