Parboiling process, rice

White rice, in comparison to rough or brown rice, is low in vitamin content (cf. Table 15.38) and in minerals. A nutritionally improved product may be obtained by a parboiling process, orginally developed to facilitate seed coat removal. About 25% of the world s rice harvest is treated by the following process raw rice — steeping in... 

In the United States, there are several patented methods of parboiling. When parboiled, the rice retains more minerals and vitamins (some of the minerals and vitamins present in the bran and embryo move into the grain proper), breaks less in subsequent processing, and stores better. 

Parboiled (converted) rice—Rice that is soaked and parboiled converted) contains much more thiamin than rice that is milled from the raw state. The parboiling causes the thiamin and other water soluble nutrients to move from the outer layers to the inner layers of the rice kernel. As a result, fewer of them are removed in the milling process. 

The three principal domestic uses for rice in 1988 were direct food (61%), processed food (18%), and beer (20%). The direct food use figure includes the conventional white milled rice plus specialty rice products (parboiled, precooked, aromatic, brown, and prepackaged mixes) shipped directly from the rice mills. The specialty products account for approximately one-fifth of the direct food use. Approximately two-thirds of the direct food use rice is ultimately distributed to consumers through retail outlets and one-third through food service outlets. 

Figure 28.13 shows the conventional production process of parboiled rice that is currently operated in some factories in Thailand and some other countries. After steaming, parboiled paddy is dried to a moisture content of 39% (d.b.) (28.5% [w.b.]) by a hot-air fluidized-bed dryer, tempered (no. 5) and dried again in the fluidized-bed dryer to 21% (d.b.). Below this, an LSU dryer is used to dry the paddy to a safe-storage moisture level. 

FIGURE 28.13 Conventional parboiled rice process production. (1) Soaking tank (2) steamer (3) ambient air ventilator (4) fluidized-bed dryer (5) tempering and air ventilation unit (6) LSU dryer (7) storage bin. (From Rordprapat, W. et al., J. Royal Inst. Thailand, 30,363,2005.)... 

FIGURE 28.16 Pasting property of parboiled paddy obtained from conventional parboiled rice production process and that of parboiled paddy obtained from superheated steam drying process. (From Soponronnarit, S. et al.. Drying TechnoL, 24, 1457, 2006.)... 

Saif, S.M.H. Suter, D.A. Lan, Y. Effect of processing conditions and environmental exposure on the tensile properties of parboiled rice. Biosystems Engineering 2004, 89, 321-330. 

Using many different samples of parboiled rice, dried in the form of paddy or processed kernels under different conditions, it was possible to cover a wide range of percentages of broken kernels. It is remarkable that the estimates from the image analysis software matched very well measurements using both the ISO norm for HRY and the visual counting (Fig. 2.11). 

Parboiling under carefully controlled process conditions can enhance the tensile strength and tensile modulus of elasticity of rice kernels. Saifet ol. (2004) found that parboiling increased the maximal tensile strength by almost four to five times compared to that of raw rice. 

Polished rice has long been associated with beriberi caused by thiamine deficiency since the polishing process reduces the content from about 4-0 to 0-7 polished rice with thiamine. 

Parboiled rice—In southern / sia, rice is parboiled before milling. /Approximately 20% of the world s rice is treated in this manner. Essentially, the process consists of soaking dry rough or paddy rice (in the hull) in warm water, steaming under pressure, and drying before milling. 

Color is another critical quality factor. It is subjectively evaluated by inspectors in a representative rice sample that has been previously milled using laboratory equipment. The color varies from desirable white to undesirable dark gray (Table 2.5). One of the most common off-colors is pink or rose, which results from the contamination of rice with red wild rice. The red pigments present in the aleu-rone-contaminated white kernels gives them a pink coloration. Parboiled rice is classified according to light or dark coloration. The hydrothermal process enhances color formation. 

Parboiling is defined as a hydrothermal process applied to rough rice in order to achieve high milling yields (Amato and Silveira 1991, Battacharya 1985). The... 

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