Bloom strength is tested by matching a standard concentration and temperature of gelatin gel, then measuring the force needed to press a plunger into it. Under these fixed test conditions, what changes between different gelatin batches is the chain length of the protein itself.
- Measure of the strength and stiffness of the gelatin: This is simply another way of naming what bloom strength is, not a factor it depends on, so it cannot be the answer to what bloom strength is proportional to.
- Density: Since the bloom test uses a fixed concentration of gelatin in water, differences in the bulk density of the raw gelatin powder do not carry through to change how firm the standardised gel turns out to be.
- Molecular weight: Under the fixed test conditions, gelatin with a higher average molecular weight has longer chains available to fold back into triple helix junctions during gelling. More and stronger junctions mean the gel resists deformation more, giving a higher bloom value. This is why gelatin manufacturers quote bloom strength as a practical stand-in for chain length or molecular weight when grading a batch.
- Viscosity: Viscosity is measured on the gelatin solution while it is still fluid, before any gel network has formed, so it describes a different state of the material from the set gel that the bloom test examines.
Since it is chain length, that is, molecular weight, that decides how many junction zones form in the standardised gel, the correct answer is Molecular weight.