Grain Logistics – Dr. Agr. Technologist
Rice is the staple food for more than half the world’s population (Childs 2004) and accounts for about a quarter of the world’s cereal production (FAOSTAT 2013). Some 90 percent of rice is grown in a small region in Asia and provides an average of 30 percent of the daily calorie intake to the Asian inhabitants (Bhattacharya 2011).
In contrast to other crops, rice is a difficult to handle cereal, because most of it goes to human consumption (Iguaz and Virseda 2007) and only a small portion is used as seed and animal feed. The ultimate objective of rice industry therefore is to produce high quality rice.
As the whole grain is the preferred form that consumers demand, broken rice has typically only half the market value of head rice (Thompson and Mutters, 2006). The causes of rice breakage and fissuring have been reviewed by Kunze and Calderwood (2004). They concluded, that mois-ture re-adsorption by low-moisture rice is the chief cause of grain fissuring leading to breakage during the subsequent milling process. Whenever ambient conditions change and moisture sorption or desorption processes are induced, the kernel surface is in equilibrium with the surrounding conditions after short time. The moisture at the center of the kernel however changes very slowly causing moisture gradients within the kernel that come along with compressive and tensile stresses which, if exceeding material strength, lead to material failure. It is therefore crucial to minimize moisture gradients during any process step.
Due to its perishable nature, rice usually has to be dried in order to prevent spoilage as well as qualitative and quantitative losses. Because of moisture changes related to this process the final product quality however is especially sensitive to the mode of drying (Iguaz and Virseda 2007). Finding the best drying process for rice is vital in order to get a product of maximum quality, although it is challenging because of the much higher susceptibility to breakage found in rice when compared to other grains. Furthermore because of its long history of cultivation and selection under diverse environmental conditions, remarkable diversity exists in rice (Childs, 2004) that makes it even more difficult to find optimum processing conditions.
A lot of effort has been directed towards achieving this goal. Traditional air drying methods aren’t best suited because of their long drying times and high pest risks. Recirculation batch dryers may increase kernel breakage because of the huge amount of recirculation, causing additional stresses. Continuous drying systems might be the solution and offer the additional benefit of supplying a constant output to the subsequent milling process. Such drying systems usually comprise of up to three dryers and temper bins in between drying passes, holding the rice for a certain period of time to allow moisture content gradients within kernels to subside and consequently to minimize stresses and resulting fissuring. The tempering phases also eliminate moisture variations of raw rice kernels that are usually found during the harvesting period (Chau and Kunze 1982).
It could be shown by various authors, that drying conditions, drying time and tempering duration have a significant effect on head rice yield (Nasrnia et al. 2010). In continuous drying, inlet temperatures in the range of 40-150°C do not affect the quality of cooked rice and the milling quality of paddy is well maintained (Tirawanichakul et al. 2004). Even very high temperatures above gelatinization temperature might be applied to dry raw paddy of high initial moisture content without prejudicing head rice yield (Ban 1971). Furthermore it was found, that high amounts of water can be removed during the first stage of drying, if sufficient tempering time is assured (Fendley and Siebenmorgen 2002), and high air temperatures can be applied in the first drying pass (Nasrnia et al. 2010).
Bühler drying technology represents state of the art continuous flow drying, which offers a variety of advantages when compared to conventional batch systems, where a dusty environment is created and the broken percentage is high due to constant recirculation. However the main problem of batch drying systems is the unevenness in moisture content after the drying process. The dwell time inside the dryer is the same for every single kernel in a batch, with no possibility for the operator to react on different incoming moisture contents, whereas the dwell times in a continuous system can always be adjusted. The loss in drying time while the batch is being loaded or unloaded and the increased need for supervision and labor also has to be considered. The Bühler process in contrast is carried out in several stages, allowing the paddy to rest in between the drying steps in order to prevent breakage due to moisture inhomogeneity. Variations in moisture are eliminated in the tempering bin that along with the special duct arrangement and separation of product flow, leads to a very uniform drying result. The diagonal duct arrangement and the resulting permanent alternation of air flow direction in EcoDryTM dryers furthermore significantly reduce thermal stress to paddy rice and guarantee for an absolutely uniform air distribution and velocity. All these advantages lead to an extremely fast, but nevertheless gentle drying process with a reduction of losses and breakage through stress cracking.
Taking a look at the energy consumption of the continuous drying solutions offered by Bühler, it is recognizable that due to the special features, there is a huge potential of savings. The possibility of up to 25 percent less energy expenses is given by the diagonal duct arrangement, the product flow separation, the design of the discharge system, the optional moisture control system, the adapted process control (such as the adjustment of the dwell times), the fitting dimensioning of the dryer, the appropriate fans and heating unit as well as the insulation of the dryers.
Bühler dryers allow for continuous drying of large quantities of grain without stopping, are de-signed for direct or indirect heating and are low in operating costs. Even moist dust can be re-moved by highly efficient central separators. The formation of residues is prevented thanks to a self-cleaning effect provided by jolting emptying that also guarantees for consistent dwell times and an uniform mass flow as well as discharge in all drying and tempering steps. The advantage of Bühler technology is the continuous mass flow even in the tempering bins while competitor solutions lead to an uneven discharge and residues within the bin. Besides a two-stage drying for raw paddy, comprised of two continuous-flow column dryers and one tempering bin in between, a three-stage solution including three dryers and two tempering bins is also available for drying of parboiled paddy. Due to the high moisture contents even higher air temperatures can be applied without deteriorating product quality.
In conclusion, it is obvious that 0 percent brokens and cracks is not achievable with any existing drying solution , but coming closer and closer to a nondissipative drying process must be the target for every rice miller. This is the idea all developments and inventions offered by Bühler are based on – always with an eye on the total cost of ownership.