Fat bloom three weeks after moulding on a 70% dark bar
A 900 kg batch of 70% dark chocolate (Peruvian mass, cane sugar, 11 kg added cocoa butter per 100 kg) leaves a three-roll refiner at D90 20 µm and is conched for 16 hours at 62 °C. It then runs through a continuous tempering machine. The operator reports an outlet temperature of 28.6 °C against a target of 31.0–31.5 °C, and viscosity measured at 40 °C has risen above the batch specification. To loosen the mass he adds 40 kg of 45 °C chocolate from the holding tank and moulds immediately. Moulds are vibrated for 20 seconds, pass a cooling tunnel whose first zone runs at 8 °C for 14 minutes, and are packed at 21 °C. The cocoa butter was delivered nine months ago and stored in an unheated warehouse that reached 27 °C in summer. Three weeks later, 30% of bars show grey streaking and dull top surfaces; a few show finger marks. Snap on the unaffected bars is clean and the eating texture is otherwise good.
1. Explain the most likely mechanism behind the bloom and rank the contributing factors.
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Primary mechanism: the temperament was destroyed by dilution. At 28.6 °C the machine was over-cooled, so the mass was stiff and rich in crystallising fat - hence the viscosity rise. Adding 40 kg at 45 °C (about 4.4% of batch) pushed a portion of the mass above the melting point of Form V (≈33–34 °C for cocoa butter), melting the stable seed crystals that had just been formed. The moulded chocolate then set largely from unstable Forms III/IV, which convert over two to four weeks to Form V with a volume change that pushes fat to the surface. Thirty per cent in three weeks with a clean snap elsewhere fits this exactly: the bloom is a surface fat-polymorph effect, not a texture failure of the whole bar.
Ranking:
- Dilution with 45 °C chocolate after tempering - decisive.
- Tempering outlet below target, so the operator had to intervene at all - root cause; check the temper index and the machine's set points.
- Cooling tunnel first zone at 8 °C for 14 minutes - very aggressive; it can set an unstable shell before the core contracts, and adds stress.
- Nine-month-old cocoa butter stored at 27 °C - a minor factor; it raises the risk of VI crystals and inconsistent tempering, but a correct temperer compensates.
Finger marks are a separate handling/hygiene point, not a cause of the streaking.
2. What do you do with the 900 kg now, and what are the limits of that decision?
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First, confirm the diagnosis. Fat bloom melts on contact and recedes when the bar is warmed to 30–32 °C; sugar bloom feels gritty and dissolves in a drop of water. A quick check under a warm fingertip and a water-drop test on two bars is enough. Streaking on the top face with clean snap and normal texture points to fat bloom, which is cosmetic and safe.
Rework. Melt the affected bars and the remaining stock fully (45–50 °C, holding long enough to melt all VI crystals), re-temper correctly, re-mould. This is legitimate and standard; bloomed chocolate is not waste.
Limits to respect. Re-melted chocolate accumulates heat damage: viscosity rises, some volatile aromatics are lost, and repeated cycles can dull the flavour of a fruit-forward Peruvian profile. Cap rework at roughly 10–20% of a fresh batch where flavour is critical, or run the whole 900 kg as a single re-melted lot and taste it against a retained reference before deciding whether it can still carry the origin claim. Do not re-temper into a premium single-origin line if the panel finds the fruit has flattened - move it to an everyday or inclusion bar instead. Record the rework in the batch traceability file.
3. Set out the process controls and target values you would put in place so this does not recur.
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Put numbers on each control point and make the operator's intervention rule explicit.
- Cocoa butter intake: reject or segregate butter over six months old or stored above 25 °C; keep it at 18–20 °C. Older butter and slow-cooled butter need longer melting and more seed.
- Tempering: target outlet 31.0–31.5 °C for dark; verify with a temper meter, not by eye - record a temper index per batch. Check the set points: 28.6 °C suggests the reheat stage is underperforming or the probe is lagging.
- Viscosity: define a working range at 40 °C. If it exceeds spec, the corrective action is to adjust tempering temperature or add fully tempered mass within a narrow window - never warm chocolate above 33 °C.
- Cooling tunnel: first zone 10–12 °C, then 15–17 °C, total 20–25 minutes; avoid the 8 °C shock.
- Moulding and packing: moulds at 28–30 °C; vibrate briefly; pack only when the bar core is below 20 °C to avoid condensation.
- Storage: 16–18 °C, below 50% RH, no thermal cycling; the 21 °C packing room with summer swings is a bloom risk in itself.
- Release test: hold a sample of every batch for four weeks at 20 °C and inspect before shipping a new recipe or a new temperer setting.