Mould inside sealed rusk bags: the monsoon complaint at a Nagpur bakery
A Nagpur bakery supplies rusk and pav to 40 retail outlets. Rusk leaves the oven at about 6.5% moisture, is cooled on open trays for five hours in a room at 31 °C and 84% relative humidity, and is then packed warm at around 40 °C into 40-micron plain PP bags of 200 g. No preservative is used. The printed label claims a 60-day best-before. This monsoon, 14 of the 60 outlets have reported white and green patches on rusk inside bags whose seals are intact, mostly at 18–24 days, and two have reported a musty smell. Quality control has checked the seals (sound), the flour (fine) and the oven (reaching 220 °C), and found nothing wrong. The owner wants to add 0.3% calcium propionate to the dough and carry on as before. The unit also has a 5,000-pack order from a railway caterer due in ten days.
1. The bags were sealed and the oven was working. Explain where the mould came from and why it grew.
Read the worked answer
The mould did not enter through the seal - it grew from inoculum already on the product, using water trapped inside the pack.
- Water activity, not moisture percentage, is what governs mould. Rusk at 6.5% moisture sits at roughly aw 0.70–0.72. Bakery moulds - Aspergillus, Penicillium, Eurotium - are inhibited only below about aw 0.65. At 0.72 the product is inside the growth range for xerophilic moulds, given enough time; 18–24 days is exactly the timescale you would expect.
- Packing warm in humid air is the trigger. Air at 31 °C and 84% RH sealed inside the bag with product at 40 °C cools after packing. As the trapped air cools, its relative humidity climbs towards saturation and condenses on the rusk surface, creating local wet spots. Those spots are where the patches appear - often on the same face in every pack, which is a classic sign of condensation rather than random contamination.
- Five hours of open-tray cooling in that room adds to it. Warm product in humid air picks up surface moisture even as the centre stays warm, setting up the moisture gradient that drives condensation later.
- Intact seals actually confirm the diagnosis. A post-pack contamination route would show random, seal-adjacent or handling-related patterns; uniform internal growth on a moisture gradient points to pre-pack inoculum plus in-pack water.
The oven and the flour were never the problem, which is why checking them found nothing.
2. Evaluate the owner's plan to add 0.3% calcium propionate. Will it solve the complaint?
Read the worked answer
It will help at the margin and will not solve the problem.
- Propionate is a weak mould inhibitor at this water activity. It is most useful against rope-forming Bacillus in bread and is far more effective at lower pH than in a low-acid rusk. At aw 0.72 the product is already in the mould growth window; a preservative slows the rate, it does not close the window.
- It is a hurdle, not a fix. Adding a preservative while leaving the moisture, cooling and packing practice unchanged means the mould appears at perhaps 35 days instead of 20 - still well inside the claimed 60-day shelf life, and still a complaint.
- The dose must be legal and labelled. Check the permitted preservative and its maximum level for bakery products in the current FSSAI regulation before use, and declare the class II preservative on the label. Propionate also carries a slight odour and taste at higher doses.
- The 60-day claim remains unsupported. A preservative does not create shelf-life evidence; only a storage trial does.
The root cause is free water and a warm-pack operation, both of which are cheap to fix. Spending on a preservative before fixing them adds cost without removing the risk.
3. Recommend the lowest-cost corrective actions, say how you would verify them, and advise on the 5,000-pack railway order.
Read the worked answer
Corrective actions, in order of cost-effectiveness
- Bake to a lower final moisture - 4.0–4.5% instead of 6.5%. Rusk is a twice-baked product and tolerates this without quality loss. This alone drops aw below 0.65 and is the single biggest lever.
- Cool in a closed, dehumidified or air-conditioned room to a core temperature of ≤30 °C before packing. Space the racks rather than stacking trays; keep cooling-room RH below 60%. A portable dehumidifier is a modest capital cost and is far cheaper than a recall.
- Pack only at room temperature, never warm. Consider a moisture-barrier film (metallised or a BOPP/PE laminate) for the monsoon months, and a desiccant sachet where the retailer's shelf conditions are unknown.
- Hygiene the cooling room. Air-settle plates will identify the inoculum source; clean and dry trays, replace wooden surfaces, keep the cooling room closed off from the packing hall, and control pests.
Verification: hold three production lots, 20 packs each, stored at ambient and at 37 °C, and test at 0, 7, 14, 21 and 30 days for moisture, aw, visible mould and sensory. Only a clean 30-day result at 37 °C (approximately 60 days at ambient) justifies printing a 60-day best-before.
The railway caterer order: accept it only if corrective actions 1–3 are in place on that run and the customer accepts a shorter, validated best-before (30 days) for the first lots. A mould complaint from a caterer feeding hundreds of passengers is far more expensive than a delayed or reduced order. Do not ship a lot you know is at risk, and reprint the label to a shelf life the plant has actually demonstrated.