May 07, 2015

07/05/2015: Making food go further - where saving cracked rice could help

by Dr Ye Aung, rice technician and consultant

First published in Milling and Grain, March 2015

We face urgent challenges as we move ever closer to 2050, when it is predicted that the world’s population will reach more than 9 billion and our growing global population will face food shortages if we cannot double food production. An important contribution to this effort can come from reducing food waste. Improved food production processes and innovations in technology must play a part in ensuring that from field to fork, food wastage is significantly reduced.

In terms of meeting this growing demand, the outlook does not look promising in light of prevailing circumstances around the world. Drought areas are expanding rapidly, while natural resources are drying up faster than expected. Further still, the denial of global warming by some means that we do not have a clear picture as to how urgently we need to act.  However, organisations such as the International Rice Research Institute are playing a pivotal part in the development of high yielding, drought resistant rice varieties.

Under these circumstances the only sensible response is to work to save food throughout the production process. Food saving is not only a matter for the future, but a thing we must turn our attention to now.
Image: Jim Sage
If we consider today’s post-harvest handling, at every step there are tremendous losses involved in turning crops into food. In the rice milling industry alone, the yield of finished rice can be as low as 40 percent, with 30 percent broken rice resulting as a by-product. However, broken rice can still be processed into usable rice, with specific characteristics, with the help of state of the art technology. Bühler is just one company at the forefront of producing top quality reconstituted rice that is indistinguishable from the natural product.
In terms of the overall process about 15 percent of rice is lost, as a raw material, when it makes its way to the milling facility. A further 20 percent is lost due to moisture problems giving a total loss amounting to about 35 percent just in raw material form. Therefore broken rice production in the milling process should be regarded as a serious issue. Milling machines are generally blamed for breaking rice but, with the right technology, the cracking of rice can be minimised. Rice is a delicate natural material and should be treated as such.

Cracked grains are the product of excessive breaks, which are formed in the grains by the drying operation and during storage.  Grain is often cracked under the influence of stresses, namely 1) moisture stress, 2) thermal stress, and 3) mechanical stress. 

Mechanical stress is easily understandable and happens during milling.  Moisture stress is created by the accumulation of moisture on the grain surface or retained within the grain itself.

Thermal stress is generated by heat - either by internal heating (the respiration process) or by external heat (during drying).  Grains are still alive even after they have been cut from the plant and they will continue to breathe.  Like other living things they produce heat, carbon dioxide and moisture.  If this heat and water are not taken away immediately they create stresses in the grains, which lead to cracking. 

Freshly harvested paddies contain a lot of external moisture that must be removed immediately.  In storage paddy rice can accumulate heat by the respiration of grains. This is harmful - not only in terms of the grain cracking but for other qualities too. Thus effective removal of heat during storage is essential.

If the cracking of rice cannot be avoided, cracks that form during the drying process should be minimised at the very least.  There is a balance between the damage that might be done by the heating involved in drying the grain and the damage done by too much moisture. The two extremes are demanding and the drying work should be carried out with extra care, particularly when drying the grain’s interior.  Removing moisture from the peripheral parts of the grain is relatively quick and carried out using a higher temperature. 

However, heating the inner parts of the grain requires more heat, which risks burning the surface and therefore the temperature should not be too high. This temperature control will mean that the rice is exposed to heat for longer, during which time the moisture is able to expand within the rice grain.

Having energy, warm moisture moves along the passages exerting pressure against the cohesion of the rice cells. Therefore the heat should not be too great lest the movement of moisture be too fast, which could immediately turn the grain’s internal passages into cracks. In this scenario heating should be halted to slow the moisture-flow. If heating  continues it could create turbulence since moisture moves from inner to outer where heat comes from outer to inner.

Turbulence in the passages could weaken the rice cells’ integrity and can quickly split them.  Therefore, heating should  stop at the correct time – when the heat  reaches  the internal moisture.  Then the stoppage time should be long enough to stop the flow of moisture completely.  This kind of interval is usually called ‘tempering’.
Image: Lara604
Therefore, the drying process is carried out in two stages with different settings for external and internal moisture removal. In an experiment, just putting tempering bins into the drying sequence slashed daily broken turnover significantly. Air temperatures were also reduced in the later phases of drying. 

Apart from stress cracks, grain suffers from another kind of crack - ignition. This  is activated by humidity pressures inside the grain and is prevalent when the internal humidity pressure is not equal to the ambient humidity pressure. Grains release moisture into the atmosphere when the latter has lower humidity. 

A reversed scenario takes place when grain has lower humidity than the ambient atmosphere.  If this action repeats frequently, cracks appear in the grains.  This often happens during storage when the stock is affected by respiration heat and released moisture.  This activity will stop when the level of humidity on both sides is the same.   

The moisture content of the grain at that time is called ‘equilibrium moisture content’ (EMC) and this condition can be created by ventilation at correct times and with correct settings.  Cracked grains are easily broken, even by the slightest impact of milling machines. Therefore it is crucial that when designing a plant the right equipment is carefully selected and specified by expert consultants.

If we are able to minimise (if not prevent completely) the cracking of grains, our world will benefit greatly. If we can save a further one percent of the total grains harvested annually from cracking, we will gain an extra 6 million tonnes of milled rice out of the 600 million tons of paddy presently produced every year. 

Fortunately, innovative technology is playing an important role in minimising breakages during processing. It is also playing an integral role in converting any broken or cracked grains into other useable forms: ground rice, used as an ingredient in other products, and extruded fortified rice grains with an appearance like that of natural rice. Saving cracked rice saves food. 

Read the magazine HERE.

The Global Miller
This blog is maintained by The Global Miller staff and is supported by the magazine GFMT
which is published by Perendale Publishers Limited.

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