Drying 300 Tonnes of High-Moisture Paddy Before It Spoils
An 8 t/h paddy mill near Sangrur takes in 300 t of PR 126 paddy at 19.5% moisture in the last week of October. Its only dryer is a batch LSU type of 6 t capacity, operating at 42 °C air temperature, taking about 8 hours to bring a batch from 19.5% to 14%, plus about 1 hour for emptying, refilling and cooling between batches. The mill's godown is still full of last season's stock, and the trader who supplied the paddy wants either immediate payment or delivery within 10 days. Ambient conditions are 30-33 °C and 80-90% RH, and it rains on two of the ten days. Normal milling of 14% paddy gives 63% head rice, 8% bran and 18% broken. The miller suggests drying at 60 °C in a single pass to save time.
1. Calculate whether the existing dryer can handle 300 t within the trader's 10-day window, and set out the realistic options.
Read the worked answer
Capacity calculation
- One cycle = 8 h drying + 1 h turn-around = 9 h, so at most 2 full batches per 24 h of continuous running (24 ÷ 9 = 2.7, but refill and labour rarely allow three).
- Throughput = 2 × 6 t = 12 t/day; allowing three batches on good days, about 15-16 t/day.
- 300 t ÷ 15 t/day = 20 days of continuous operation, and about 25 days at 12 t/day. Rain on two days makes it worse because the dryer air cannot be humidified down and intake RH is already 80-90%.
- In 10 days the mill can dry roughly 120-160 t. It faces a shortfall of at least 140 t.
Options, in order of practicality
- Rent or borrow a second dryer, or send part of the lot to a nearby custom drying plant; transport cost is far below the loss on spoiled paddy.
- Sell the surplus wet paddy immediately to a larger mill that has continuous drying capacity, accepting a discount; a known discount is better than an unknown spoilage loss.
- Sun-dry on tarpaulin in thin layers during the dry daylight hours for the portion that will be parboiled anyway, and finish in the LSU dryer.
- Keep undried paddy under forced ambient aeration only as a short holding measure; aeration slows heating but does not remove 5.5 percentage points of moisture.
- Renegotiate the 10-day clause with the trader in writing, showing the drying log as evidence.
Decision: commit to a combination of 1, 2 and 5 today. Doing nothing and hoping the dryer keeps up is the only option that guarantees a loss.
2. Why is the miller's single-pass 60 °C suggestion a false economy? Explain the kernel physics and put a rupee figure on the risk.
Read the worked answer
Why it fails
- A 19.5% moisture kernel dried fast at 60 °C develops a steep moisture gradient: the surface shrinks while the centre is still wet. The resulting tensile stress exceeds the kernel's strength and produces fissures, which shatter during milling.
- At high temperature and high moisture, surface starch begins to gelatinise and the kernel case-hardens, further slowing internal moisture movement and increasing the gradient.
- The result is a higher broken percentage and lower head rice, even though the moisture meter reads 14% and the paddy looks dry.
Better practice
- Two-pass drying: bring the paddy to about 16-17% at 42-45 °C, temper for 4-8 hours in a bin so moisture equalises, then finish to 13.5-14% in a second pass.
- Limit each pass to roughly 1-2 percentage points of moisture removal and cool slowly in the bin, not by sudden cold air.
Rupee estimate
- 300 t at 63% head rice yields 189 t of head rice. If careless drying drops head rice by 5 percentage points, about 15 t moves from head rice to broken.
- If head rice sells at about Rs 45,000/t and broken at about Rs 25,000/t, the loss is roughly Rs 20,000 × 15 t = Rs 3,00,000, which is many times the cost of extra drying time or rented dryer capacity.
- Add the risk of rehydration if the over-dried, case-hardened grain is exposed to humid October air.
3. After drying, how should the paddy be stored and monitored, and what would make you reject a bag?
Read the worked answer
Storage specification
- 13.5-14% moisture is acceptable for a few weeks at 30 °C, but for stock held more than two to three months target 13% or lower, because safe moisture falls as temperature rises.
- Cool the grain to ambient before bagging; bagging hot grain drives moisture to the bag surface and causes condensation and mould.
- Stack on wooden pallets at least 10-15 cm off the floor and 50 cm from walls, with aisles for inspection and a first-in-first-out issue rule.
Monitoring
- A calibrated moisture meter checked against the oven method, sampled with a probe at several depths in each stack.
- Weekly temperature checks with a probe; a rise of 2-3 °C above ambient indicates heating and incipient mould or insect activity.
- Smell and appearance checks: musty odour, dull or discoloured kernels, webbing or live insects mean rejection of that bag.
- Free fatty acid and insect counts on retained samples for longer storage; FFA rises as bran lipids are hydrolysed, and high FFA and off-odour go straight into the oil and bran fractions.
Reject a bag if it is visibly mouldy, has a musty or sour smell, shows heating, contains live insects or webbing, or its moisture is above 14.5% after drying. Segregate it, do not blend it into sound stock, and record the quantity and reason.