The Foumban Tomato Dryer: Mass Balance, Defects and a Bad Idea from a Member

Principle. Dry matter is conserved during drying; only water is removed.

Dry matter in the fresh fruit = 600 kg x (1 - 0.94) = 600 x 0.06 = 36 kg.

At 12% moisture the product is 88% dry matter: mass = 36 / 0.88 = 40.9 kg

At 10% moisture the product is 90% dry matter: mass = 36 / 0.90 = 40.0 kg

Interpretation

• The extra drying costs only about 0.9 kg, roughly 2% of output, so the buyer's stricter specification is cheap to meet in mass terms.
• What it buys is a safety margin. Water activity, not moisture percentage, governs microbial growth. A product at 12% moisture equilibrated at 25-30 C in a humid Douala store will pick up moisture from the air; it can drift towards 0.65 water activity or above, where xerophilic moulds grow. A product at 10% starts further below the threshold of about 0.60 and tolerates more moisture pickup during six months of storage.
• The 15:1 fresh-to-dry ratio means the 40 kg of product represents the entire 600 kg harvest; the economics depend on the dried price, not on the fresh price.
• The calculation is theoretical. Real yield is lower after trimming damaged tissue, peeling, and leaching losses during blanching, typically 5-15%. A realistic plan should budget for about 34-38 kg of saleable product, and the cooperative should weigh the output rather than assume it.

Diagnosis

• Browning. Two mechanisms. Enzymic browning: cutting exposes polyphenol oxidase to oxygen, producing brown melanins within minutes. Non-enzymic browning: three days of slow drying at ambient temperature allows Maillard reactions and ascorbic acid oxidation, which also dull the red colour.
• Mould. Slow open drying means the slices spend 24-48 hours in the 0.85-0.95 water activity range, ideal for mould growth; night-time rehydration and dew make it worse. Aspergillus and Alternaria species are likely, and Alternaria toxins are a real concern in tomato.
• Insects. Open mats with no screening allow flies, beetles and dust; eggs laid on the wet surface hatch during the long drying period.

Process design

1. Sort and wash. Remove damaged, over-ripe and mouldy fruit before slicing. Slice uniformly, 5-8 mm, so all pieces dry at the same rate.
2. Pretreat. Steam-blanch 3-5 minutes to inactivate polyphenol oxidase and expel surface air, or dip in 0.2% potassium metabisulphite plus 1% citric acid. Sulphite controls browning and mould but must be declared on the label and is a concern for asthmatics; if the buyer requires sulphite-free product, rely on blanching and speed.
3. Dry fast at the start. Load the cabinet dryer at 2-3 kg per square metre, set 55-60 C with the vents open for the first two hours to remove free water, then 50 C with good air flow. Target 8-12 hours to 10% moisture, not three days.
4. Equilibrate. Seal the dried slices in a bin for 24 hours so moisture equalises, then re-check with a moisture meter and reject any batch above 10%.
5. Package and store. Seal in metallised or high-barrier pouches, label with batch and date, and store below 25 C away from light.
6. Enclose the process. Insect-proof mesh of 1 mm or finer on all openings, and strict cleaning between batches.

Verification. Keep records of dryer temperature, drying time, final moisture and a weekly mould check; send one batch for microbiological and moisture testing before the first delivery.

The two errors in the proposal

1. Sunlight does not sterilise. Ultraviolet light acts only on exposed surfaces; it is blocked by shadow, by the slice itself and by packaging. The temperatures reached on a mat are inconsistent and drop at night. Spores survive and germinate as soon as the moisture returns during equilibration or storage.
2. Drying does not remove mycotoxins. Alternaria toxins such as alternariol and tenuazonic acid, and aflatoxins from Aspergillus flavus in badly stored material, are heat stable and survive drying, boiling and cooking. Visible mould also means that hyphae extend well beyond the colony you can see.

Mixing is adulteration. Diluting a contaminated batch does not remove the hazard - it spreads a low, unmeasured dose across the whole consignment, making the entire lot unsaleable and potentially harmful. It is also illegal in most jurisdictions, and the cooperative's reputation would not recover if a Douala buyer detected it.

Correct fate of the mouldy fruit. Reject and discard it, or compost it. It must not enter the food chain. Sorting at the point of harvest is the control point: damaged and over-ripe fruit should be diverted before slicing, and the mouldy slices should be visibly discarded in a marked bin so that the practice is seen to be enforced.

Answering the economic argument

• The unsold 360 kg, at 250 FCFA/kg, is worth about 90,000 FCFA if it could have been sold fresh - and it could not. As dried product it might be worth more, but a single rejected consignment destroys the contract, which is worth far more than 90,000 FCFA over six months, and closes the market for every member.
• A consignment of 40 kg at, say, 3,000 FCFA/kg is 120,000 FCFA of revenue at risk, plus the cost of the whole harvest and the drying labour.
• The rational comparison is the cost of losing 10-15% of the fruit to sorting versus the risk of losing the entire contract and reputation. Sorting wins overwhelmingly.
• The cooperative should also invest in a moisture meter, thermometer and training, and consider a faster process or a smaller batch size so that over-ripe fruit is dried the same day.