Soft matter / 02
How firm is a gummy bear
Ask how firm a gel is and you need a definition of firm. The food industry has used the same one for a century, and it is refreshingly physical: push a plunger into the gel and see how hard you have to push.
The Bloom test
The test is named after Oscar T. Bloom, who patented the apparatus in the 1920s. The modern version is standardised, and the standardisation is the point: gel strength depends on so many variables that a number only means something if everything else is fixed.
A gelatin solution is prepared at a fixed concentration, conventionally 6.67 percent by mass, matured cold at around ten degrees for a fixed time, typically overnight. A plunger of specified diameter is then pressed into the surface, and the force needed to depress it by four millimetres is recorded in grams. That number is the Bloom value.
Commercial gelatins run from roughly 50 Bloom at the soft end to around 300 at the firm end. Confectionery and dessert grades commonly sit somewhere in the 150 to 250 region, though this varies by manufacturer and application.
What the number is really telling you
Bloom correlates with the average molecular weight of the gelatin chains. Longer chains participate in more junction zones each, tie the network together more effectively, and so give a firmer gel at the same concentration.
That has practical consequences beyond firmness. Higher Bloom gelatin generally sets faster, melts at a slightly higher temperature, and lets you use less of it for the same result, which matters when the gelatin itself contributes flavour. Lower Bloom grades are chosen where a softer, more yielding set is wanted, or where a lot of protein is needed for reasons other than gelling.
Because Bloom is defined at a fixed concentration, it describes the gelatin, not the product. A 250 Bloom gelatin used at half strength gives a soft jelly. The number is a property of the ingredient.
The two ways to make a gummy bear firm
A dessert jelly and a gummy bear can be made from similar gelatin. What separates them is mostly water.
A wobbling dessert jelly uses a low percentage of gelatin in a lot of water. A gummy sweet uses considerably more gelatin and far less water, with much of the remainder being sugar and glucose syrup. The finished sweet is dried down further after moulding, which concentrates everything again.
Less water per unit of network means the junction zones are closer together and the chains between them are shorter and more tightly packed. The result is a gel you have to chew rather than one that collapses under a spoon. Same chemistry, different water content, and the mechanical difference is enormous.
Why the same firmness can feel different
Bloom captures one number. Texture is not one number.
Two gels can require the same force to indent and still feel completely unalike, because they differ in how they fail. A gelatin gel yields gradually and springs back: pull it and it stretches a long way before tearing. An agar gel of the same measured strength snaps. In rheological terms they differ in strain at fracture, and that is what the mouth registers as the difference between chewy and brittle.
This is why confectioners blend gelling agents. Gelatin brings elasticity and the melt. Pectin brings a cleaner, shorter bite and works in vegan formulations. Starch gives the soft body of a jelly bean centre. Carrageenan can be tuned between brittle and elastic by choosing the type and the salt. Combinations exist because a single agent rarely gives the whole texture wanted.
The things that quietly ruin a set
Several common ingredients interfere, and the reasons are worth knowing.
Fresh pineapple, kiwi, papaya, figs and fresh ginger contain protease enzymes. Gelatin is a protein, and proteases cut proteins. Add any of them fresh to a gelatin jelly and it will not set, or will set and then slump as the enzymes work. Canned pineapple is fine, because the heat of canning denatures the enzymes.
Acid weakens a gelatin set, so very sharp fruit preparations usually need more gelatin than a neutral one.
Boiling can degrade gelatin, cutting chains shorter and lowering the effective Bloom. Dissolve it warm rather than hot.
Alcohol at high concentrations interferes with the network, which is why strong jelly shots need proportionally more gelatin than the same volume of juice.
You can feel the consequences of firmness directly on the interactive jelly: the firmness control changes how strongly the network resists being pulled out of shape, which is the same axis the Bloom test probes.