Paddy Drying Fissures: Why They Form and How to Prevent Them

Moisture control in harvested paddy is a critical factor for ensuring long-term safe storage and improving grain quality. However, during paddy drying process, improper settings of parameters such as temperature, drying rate, tempering temperature, and duration can easily cause drying fissures (commonly known as fissured/broken kernels). This not only increases the broken rice rate during milling and drastically reduces the Head Rice Yield (HRY), but also leads to direct economic losses.

What is a Paddy Drying Fissure and When Does It Appear

Drying cracks (fissures) are internal micro-fractures within the rice kernel. Invisible from the outside, they represent physical traces of the grain being “pulled apart from within” by stress exceeding its internal limit. They are typically only discovered when kernels shatter into broken rice during milling.

Paddy drying fissures almost never appear during the drying process. All fissures form during the cooling phase after drying stops.

• Most fissures begin to develop 2 hours or more after drying ends.
• Some appear up to 9 hours later after the dryer stops.
• All fissures emerge within 24 hours.
• Under rapid drying conditions, multiple fissures develop in the later stage.

Formation Mechanism: Moisture Gradients & Glass Transition Theory

Traditional drying theory holds that fissures are primarily caused by moisture gradients and internal stresses resulting from uneven rates of moisture loss between the inner and outer layers of rice kernels. Modern drying research explains this via the Glass Transition Temperature (Tg) of starch.

Starch, the main component of the rice endosperm, is a partially crystalline and partially amorphous mixture, where the amorphous portion can switch between two physical states:

• Glassy state — hard, high elastic modulus, low expansion, slow water diffusion.
• Rubbery state — soft, low elastic modulus, high expansion, fast water diffusion.

Note: The glass transition temperature (Tg) has a linear relationship with moisture: higher moisture content lowers Tg.

Phase Transition and Physical Property Changes

The main component of paddy endosperm is starch, whose phase transition is determined by wet-basis moisture content (MC) and temperature. In the rubbery state, paddy exhibits high elasticity, low hardness, a large expansion coefficient (about 5 times that of the glassy state), and fast moisture diffusion. When transformed into the glassy state, it exhibits high hardness, poor elasticity, a low expansion coefficient, and slower moisture diffusion.

Phase Differences and Stress Concentration Caused by Moisture Gradients

During hot-air drying, wet paddy surfaces heat up and dehydrate rapidly. As surface moisture drops, the outer layer transforms from the rubbery state to the glassy state first, while the inner core remains in a high-moisture rubbery state. The glassy outer layer tends to shrink while the rubbery core tends to expand, subjecting the outer layer to tensile stress and the inner layer to compressive stress.

Since the tensile strength of paddy is only 1/14– 1/7of its compressive strength, an excessive drying rate and moisture gradient will cause tensile stress on the outer layer to exceed its mechanical strength limit, resulting in fissures.

Impact of Tempering and Cooling

If the tempering temperature stays below Tg: the outer layer drops into the glassy state while moisture is still migrating outward. If the moisture gradient across that boundary is large enough, fissures form.

If the tempering temperature stays above Tg: the moisture gradient disappears before the glass transition happens, so no cracks form — and the batch can then be cooled below Tg safely.

If the paddy grain is cooled straight below Tg with no tempering at all: the moisture gradient at the moment of glass transition is even larger, making fissuring more likely.

Losses Caused by Paddy Drying Fissures

• Severe Reduction in Head Rice Yield (HRY): Fissured paddy grains break easily during husking and milling, directly reducing overall milling yield and Head Rice Yield (HRY).
• Loss of Commercial Value and Profitability: Increased broken rice lowers rice grade and market value, causing direct financial losses for processors and farmers.
• Higher Susceptibility to Breakage: Paddy grains with micro-fissures lose structural resistance, becoming highly sensitive to secondary breakage during handling, transit, and storage.

How to Prevent Paddy Drying Fissures

Experimental data shows that moisture gradients have a far greater impact on fissures than temperature gradients. Therefore, the core of fissure prevention lies in controlling moisture gradients and implementing proper tempering processes:

Strictly Control Dehydration (Drying) Rate: Avoid excessively high single-pass moisture reduction rates. If the drying rate exceeds the maximum moisture gradient limit, irreversible fissuring occurs, and subsequent tempering will fail to recover the HRY.

Implement High-Temperature Tempering: Dried paddy must undergo thorough tempering at temperatures above Tg. This allows internal moisture to migrate outward and eliminates the moisture gradient before cooling, effectively preventing fissuring.

Avoid Rapid Forced Cooling: Never cool paddy directly below Tg immediately after drying without first dissipating the internal moisture gradient through tempering.

Recommendations for Paddy Drying

To maximize processing quality and HRY while maintaining high drying efficiency, the following process parameters are recommended during paddy drying:

Multi-Stage Drying Rate Control

• High-Moisture Stage: In the rubbery state, paddy exhibits high moisture diffusion and elasticity. Dehydration rates can be appropriately increased, up to 5% – 6%/h.
• Low-Moisture Stage: As paddy transitions into the glassy state, diffusion slows down. The dehydration rate must be reduced and maintained at 0.7%–1%/h to prevent severe moisture gradients.

High-Temperature Drying Combined with High-Temperature Tempering

Adopt a combination of high-temperature drying and high-temperature tempering above Tg. Higher single-pass moisture drops or longer drying times require proportionally longer tempering durations.

Utilize Modern Continuous Drying Systems

It is recommended to use industrial-grade grain drying systems equipped with specialized tempering silos, dryers, smart conveyors, and cooling systems (such as GSS Continuous Paddy Dryers or Batch Grain Dryers). These systems precisely maintain the optimal drying-to-tempering time ratio, ensuring superior grain quality.

Investing in modern drying infrastructure and adhering to optimal moisture-gradient controls not only safeguards structural grain integrity and maximizes Head Rice Yield (HRY), but directly translates into higher commercial profitability and superior market quality for every batch harvested.