Chocolate tempering and bloom: why good chocolate turns grey and how to stop it

Chocolate that snaps, shines and melts cleanly on the tongue, and a dull greyish bar that crumbles, can come from the same recipe. The difference is temper — the crystal form the cocoa butter sets in. Tempering decides gloss, snap, shelf life and whether the product reaches the shelf looking sellable.

Cocoa butter can crystallise in six different forms, and only one of them gives stable, glossy chocolate. Tempering is the controlled path to that form: melt fully near 45 °C, cool to ~27 °C so seed crystals form, then warm back to ~31-32 °C for dark to melt out the unstable ones.

What tempering actually does

The working window is narrow: dark chocolate is held around 31-32 °C, milk 30-31 °C, white 28-29 °C because of its higher milk-fat content. A degree or two off and the chocolate either sets soft and dull or thickens too fast to mould.

A continuous tempering machine runs that curve automatically — multi-zone heat exchange plus a screw or scraped-surface section that builds seed crystals. The point is repeatability. Hand-tempering on a marble slab works for a chocolatier doing kilos, not for a line running hundreds of kilos a shift where every batch must look identical.

Fat bloom vs sugar bloom: two opposite failures

Grey film on chocolate is not mould and not expiry — it is bloom, and there are two kinds with opposite causes. Fat bloom is unstable cocoa-butter crystals migrating to the surface and recrystallising: it comes from poor temper or from heat in storage and transport (above ~18-20 °C the fat starts moving). Sugar bloom is different: moisture condenses on the surface, dissolves sugar, then evaporates and leaves rough crystals. Its cause is cold product meeting warm humid air — the same condensation problem as a badly run cold chain.

A practical rule: if the grey film feels greasy and melts under a finger, it is fat bloom and the cause is upstream, in the temper or in storage heat. If it feels dry and gritty, it is sugar bloom and the cause is condensation. Chasing the wrong one wastes weeks.

Why the cooling tunnel matters as much as the tempering machine

Tempering sets the seed; the cooling tunnel locks it. Pull heat too fast and the surface sets before the core, trapping stress that shows up as cracks and dull patches; too slow and unstable crystals form anyway and the temper you just built is lost. A chocolate tunnel runs in zones, roughly from 12-15 °C and back up, so the product never reaches the dew point and never thermally shocks.

The detail people miss: cooling air must stay above the room's dew point, or condensation forms on the chocolate as it exits — instant sugar bloom on a perfectly tempered product. In a humid climate the tunnel and the room's humidity control are one engineering problem, not two.

Where confectioners lose money on chocolate

Three places the money goes. A moulding or coating line bought without a properly sized tempering machine — the temper drifts mid-run and a batch sets with bloom; even a few percent reject on a line doing 300-500 kg/shift becomes tonnes over a month. Skipping the cooling tunnel and setting product in ambient air — uneven set, cracks, demoulding losses. Running the workshop hot to save on climate control — above ~20 °C chocolate will not hold temper and rework piles up.

Tempering and cooling are not accessories to a chocolate line — they are the line. The depositor only shapes what the temper and the tunnel already decided.