The factors in controlling the material properties of parboiled rice are the diffusion of water and other compounds into and out of the rice grain, starch gelatinisation and retrogradation and the protein denaturation and disulphide linkage.
Diffusion is a key parameter to dictate the final quality of parboiled rice. The diffusion properties of rice depend on a number of factors including grain structure, composition, post-harvest processing, temperature and moisture content. Fick’s second law of diffusion has been widely used to determine the diffusion behaviour in grains during soaking and drying. Researchers have also applied semi-theoretical models to understand the diffusion process in the rice grain.
Gelatinisation and re-crystallisation are the major changes in rice starch that occur during parboiling. During gelatinisation the swollen starch granules melt and the phase transition of starch occurs. And in re-crystallisation process, the starch exhibits polymorphisms which contribute the final texture of cooked parboiled rice.
The extent of changes within the rice kernel during hydrothermal treatment depends on the processing variables applied during processing. The parboiled rice kernel is thicker and shorter than non-parboiled counterpart. The opaque white patches seen on the belly of the milled rice (chalkiness) is removed by parboiling. The parboiled rice has a shade of amber colour possibly due to the diffusion of husk colour into the endosperm or non-enzymatic Maillard type of browning. The parboiled rice has higher mechanical strength which makes it less susceptible to breakage during milling. The content of B-vitamins is increased in parboiled rice. The eating properties of parboiled rice are also altered than non-parboiled counterpart because it is less sticky, absorbs less water and takes a bit more time in cooking.
The severity of the time–temperature treatment adopted during different parboiling process affect the final quality of parboiled rice. For example, the colour change increases with increasing soak water temperature and increasing steaming duration as well as the increasing steaming pressure. Whereas, the hardness value decreases with the increasing soak water temperature but increases with increasing steaming time. The parboiled rice produced with high soak water temperature was reported to be less in lipid content, higher in thiamine content (Chukwu and Oseh, 2009) but experienced higher starch leaching. The thiamine content in the parboiled rice increases gradually as soaking temperature and steaming duration is increased. Whereas, the increasing steaming period decreased protein, calcium, iron and sodium but increased the fat, total ash and crude fibre content in parboiled rice. There is loss of vitamins A and C which is directly related to the severity of temperature.