Soft matter / 02
How firm is a gummy bear?
How firm is firm? The food industry has used the same answer for a century, and it's gloriously simple: push a plunger into the gel and see how hard you have to push.
Meet the Bloom test
It's named after Oscar T. Bloom, who patented the kit in the 1920s. Today's version is standardised, and that's the whole point: gel strength depends on so many things that a number only means something if everything else stays fixed.
Here's the recipe. Make a gelatin solution at a fixed strength, usually 6.67% by mass. Let it mature cold, at about 10 °C, for a fixed time, usually overnight. Press a plunger of a set width into the surface. The force needed to push it down 4 mm, in grams, is the Bloom value.
Shop-bought gelatins run from about 50 Bloom (soft) to around 300 (firm). Sweets and desserts usually use something in the 150 to 250 range, though it varies by maker and use.
What the number really tells you
Bloom tracks the average molecular weight of the gelatin chains. Longer chains join more junction zones each, tie the net together better, and give a firmer gel at the same strength.
That changes more than firmness. Higher Bloom gelatin usually sets faster, melts a bit hotter, and lets you use less for the same result, which matters when the gelatin adds its own flavour. Lower Bloom grades get picked for a softer, more giving set, or when you need lots of protein for reasons other than gelling.
Bloom is measured at a fixed strength, so it describes the gelatin, not the sweet. Use 250 Bloom gelatin at half strength and you get a soft jelly. The number belongs to the ingredient.
Gummy bear vs wobbly jelly
A dessert jelly and a gummy bear can use similar gelatin. What separates them? Mostly water.
A wobbly jelly is a little gelatin in lots of water. A gummy has much more gelatin and far less water, and most of the rest is sugar and glucose syrup. After moulding, the sweet gets dried even more, which packs everything tighter again.
Less water per bit of net means junction zones sit closer together, with shorter, tighter chains between them. So you get a gel you have to chew, not one that collapses under a spoon. Same chemistry, different water, massive difference.
Same number, different feel
Bloom is one number. Texture isn't.
Two gels can need the same push to dent and still feel nothing alike, because they break differently. Gelatin gives way slowly and springs back: pull it and it stretches a long way before it tears. Agar with the same measured strength just snaps. Scientists call this strain at fracture. Your mouth calls it chewy versus brittle.
That's why sweet makers mix gelling agents. Gelatin brings the stretch and the melt. Pectin gives a cleaner, shorter bite and works in vegan sweets. Starch makes the soft middle of a jelly bean. Carrageenan can go brittle or stretchy, depending on the type and the salt. One agent rarely does the whole job.
Set wreckers
Some everyday ingredients ruin a set. Here's why.
Fresh pineapple, kiwi, papaya, figs and fresh ginger contain protease enzymes. Gelatin is a protein, and proteases chop up proteins. Add any of them fresh and your jelly won't set, or it sets and then slumps as the enzymes get to work. Tinned pineapple is fine: the heat of canning denatures the enzymes.
Acid weakens the set, so very sharp fruity mixes usually need more gelatin than a neutral one.
Boiling can damage gelatin, cutting chains shorter and lowering its real Bloom. Dissolve it warm, not hot.
Alcohol at high concentrations gets in the way of the net, so a jelly made with a lot of it needs more gelatin than the same amount of juice would.
Feel it yourself on our jelly: the softness slider changes how hard the net fights being pulled out of shape. That's the same thing the Bloom test measures.