Mango Bar Batch Spoils Before the Fair: Drying, Water Activity and Sulphite Dose
An SHG in Malda buys 120 kg of ripe Fazli mangoes at ₹22/kg at the peak of the season. Pulp recovery after washing, peeling and deseeding is 60%. The pulp is cooked with sugar equal to 25% of the pulp weight and 0.3% citric acid, then spread about 1 cm thick on bamboo mats and sun-dried for three days. On day two the weather turns humid, around 85% RH, with a short shower, and the mats are shifted under a tarpaulin. On day three, white and green mould colonies appear on the top surface and the bars smell fermented. A hand refractometer reading of the finished bar is 62 °Brix and the moisture meter shows 18%. No preservative was used. The batch is to be packed in 200 g low-density polyethylene pouches for a local fair ten days later. The group holds a food processing licence, owns a stainless steel kettle, and has an idle hot-air tray dryer in the shed next door.
1. Using the readings given, explain why the batch spoiled and identify the three most important control points that were lost. Would using the idle tray dryer alone have saved this batch? Justify your answer.
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Why it spoiled. The bar finished at 18% moisture and 62 °Brix. That corresponds to a water activity of roughly 0.75–0.80 - low enough to stop most bacteria, but comfortably inside the growth range of xerophilic moulds such as Aspergillus and Penicillium, which grow down to Aw about 0.65–0.70. Sugar at 62 °Brix is a hurdle, not a preservative.
The three lost controls:
- Drying - still air at 85% RH for three days meant the surface stayed moist for 48 hours, and the shower added moisture back.
- Final moisture/Aw - 18% is well above the 12–15% target for a fruit bar.
- Packaging - LDPE has high moisture-vapour transmission, so even a dry bar would pick up moisture during ten days at a fair.
Would the dryer alone save it? Only partly. Forced air at 55–60 °C would dry the sheet quickly to ≤12% and pull Aw below 0.65, removing the mould's main growth condition. But the dryer does nothing about recontamination from mats, hands and dusty storage, nor about a warm loose pack. The correct answer is dryer + moisture target + hygiene + preservative + barrier pouch, not dryer alone.
2. The permitted level is 100 mg of SO2 per kg of finished product, and potassium metabisulphite (KMS) contains 57.6% available SO2. Calculate the KMS needed for a 23 kg lot of bars. State two handling precautions and the label consequence.
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Calculation.
- SO2 required = 100 mg/kg × 23 kg = 2,300 mg = 2.3 g of SO2.
- KMS needed = 2.3 ÷ 0.576 = 3.99 g, i.e. about 4.0 g for 23 kg.
Work through the logic in this order: convert the permitted ppm into total SO2 for the batch, then divide by the fraction of SO2 in the salt. A common error is to divide instead of multiply, or to forget that the limit applies to the finished product, not to the pulp.
Precautions. Dissolve the weighed KMS in a small quantity of water and mix it uniformly into the pulp before drying - never dust the dry salt onto the finished bar, as that gives hot spots above the limit. Use food-grade KMS, weigh on a balance reading to 0.1 g, and work in a ventilated area because acid plus sulphite releases SO2 gas, which irritates the lungs.
Label consequence. The pack must declare the preservative in the ingredient list (class name with theINS number) since it is an added substance; the group also needs to keep a batch record of the weighed quantity.
3. The mouldy lot is about 40 kg and the group wants to scrape off the surface growth and sell it cheaply at the fair. Give your decision with reasons, and outline the corrective actions you would insist on before the next batch.
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Decision: reject and destroy the 40 kg; do not scrape and sell. Visible mould is only the fruiting body. Mould hyphae and mycotoxins such as aflatoxin penetrate soft, moist fruit bars well beyond the surface, and scraping removes neither. Mycotoxins are heat-stable, so re-cooking does not make the lot safe. Selling it, even cheaply, exposes the group to a food safety complaint, loss of licence and reputational damage that far exceeds the value of 40 kg of bar. Divert it to compost or dispose of it as waste and record the quantity, date and reason.
Corrective actions before the next batch:
- Set a written specification: final moisture ≤12%, Aw ≤0.65, checked on every lot with a moisture meter.
- Fix a drying schedule in the tray dryer - 55–60 °C, thin loading, measured air flow - and stop relying on sun and weather.
- Add the calculated KMS dose and record the weight.
- Cover mats and use only cleaned, sanitised utensils; handlers to use gloves and caps.
- Switch to a laminate pouch with a moisture barrier, and print batch number and best-before date.