Parboiled rice is usually defined as rice which has been steeped, heat treated and dried. During the heat treatment step of parboiling, the starch in the endosperm of the rice is substantially gelatinized. The parboiling process and the resulting gelatinization of the starch have several beneficial effects.
First, rice is routinely parboiled to achieve a better milling yield (less broken rice). Less broken rice has significant economic and quality consequences; whole rice commands a higher price because whole grains are valued by consumers of rice as high quality. Upon cooking, a rice with less brokens is generally accepted world-wide as having a more pleasing appearance.
Parboiling also causes a very important second quality change, which becomes evident upon cooking. Cooked parboiled rice grains are significantly more intact and retain their natural shape as compared to non-parboiled rice. In selected rice eating cultures of the world, this is viewed as a quality improvement over non-parboiled rice.
Also, during the parboiling process, the rice grain is strengthened to impart increased resistance to the rigors of abrasive milling. (Unparboiled rice easily shatters.) The strengthening of the grain is manifest in the cooked finished product. The rice is so strengthened by parboiling that it typically takes somewhat longer to cook during preparation by consumers than does unparboiled rice. Furthermore, parboiled rice often has a firmer texture and is less sticky than unparboiled rice. Even with the cooktime increase, these changes make parboiled rice more attractive than unparboiled rice in selected cultures around the world.
Parboiling of rice apparently originated, principally, in India. In the early history of parboiling, the rough (paddy) rice was simply soaked in warm water overnight and then dried in the sun. The perceived benefit was that the rice hulls were split open and were thus easily removed from the rice kernel. In modern times, it has been realized that parboiling also provides a more nutritional rice as thiamine and other essential nutrients, which are normally present in the rice bran, migrate to the rice endosperm during the water steeping or soaking step. Since almost all rice is milled to remove the bran, this migration preserves at least some of the nutritional value initially contained in the bran. Parboiling is also beneficial since the starch in the rice endosperm is changed from a partially crystalline-partially amorphous state to a substantially amorphous state. With the starch in the amorphous state, the kernel is tougher, resulting in a higher yield of whole rice kernels after milling. As mentioned earlier, unparboiled (crystalline) rice easily shatters. Gelatinization via parboiling, simply put, is the water-assisted melting of starch granules upon heat treatment. The presence of too much or too little water when heat is applied to the starch can have beneficial or deleterious results. Another significant benefit of parboiling is that the lipase in the bran layer of brown rice becomes inactivated due to the heat treatment. This improves the shelf life of parboiled brown rice by reducing the tendency for oxidative rancidity.
As a rule of thumb, prior parboiling practices can be divided into three broad categories: the "atmospheric steaming" method, the "dry-heat" method and the "pressure-steaming" method. The atmospheric steaming method comprises soaking, draining, and steaming at atmospheric pressure, followed by drying and milling stages. The dry-heat method replaces the steaming step of the conventional method with a heating stage in which the rice is cooked in dry-hot air, hot non-aqueous liquids or hot sand prior to drying. Sometimes, the hot dry heat media are replaced by electromagnetic energy heating, such as microwave heating. In all cases with the dry heat method, the use of water or steam is avoided. The consequences of avoiding the use of water will become apparent later in the text. Finally, the pressure-steaming method comprises a low-moisture initial soak followed by pressurized steaming prior to drying and milling.
Today, commercial parboiling processes generally include the steps of: (I) soaking rough (or paddy) rice in 50.degree.-70.degree. C. water for 3-4 hours to yield a rough rice having a 30 weight percent water content; (2) draining the free water from the soaked rice; (3) applying steam heat under pressure for 10 to 20 minutes to effect gelatinization; and (4) drying the steamed rice with hot air to reduce its water content to about 14 weight percent water. The dried, parboiled rough rice is then ready for shelling (to remove the hull) and milling to remove the bran.
Parboiling has been an active topic in the patent literature. There have been numerous efforts to improve upon the basic technology. For example, U.S. Pat. No. 5,017,395 teaches an extra predrying step at an elevated temperature. U.S. Pat. No. 4,810,511 prescribes use of microwave energy for partial gelatinization. According to U.S. Pat. No. 4,361,593, the rice starch is not completely gelatinized during steaming, and a tempering step is performed under non-gelatinizing conditions to reduce subsequent rupturing. In U.S. Pat. No. 4,338,344, there is disclosed an inclined enclosed chamber where rice is cooked in hot water in a first zone at a lower end, and then is steamed in a second zone at an upper end.
Unfortunately, despite these treatments, two undesirable conditions persist: conventional processes cause parboiled rice to be yellow and to develop a characteristic "parboiled" flavor in appreciable part due to Maillard-browning effects (and also in part as a result of the effects of agents contained in the rice hull when using paddy rice). To many cultures and consumers, these conditions are objectionable. In fact, many consumers believe that the yellow color and parboiled flavor signify that the rice is old and stale. This is important because, as widely accepted in culinary arts, the first impression of a food is generally visual. That is, the willingness of a person to eat a particular food depends largely on preconceptions as to appealing color and other visual cues. Color is an influential quality attribute pre-supposed by people to be an indicator of deteriorative changes undergone by food. Aroma and flavor can also be influential upon smelling and tasting. Thus, an off-color, increases the likelihood that a food will be rejected, and this phenomenon jeopardizes the acceptability of parboiled rice amongst the majority of rice eating cultures around the world. This is so to an even greater extent if the rice has a foreign flavor. It is desired that rice color be near-white and that rice flavor be near bland and subtle.
Only a relatively small group of consumers of conventional parboiled rice accept its appearance and flavor. Indeed, the overwhelming majority of rice consumers, worldwide, eat non-parboiled rice. To our knowledge, none of the prior parboiling practices has resulted in a rice which combines the beneficial features of parboiled rice such as intact grain structure with an improved color vis-a-vis that exhibited by typical parboiled rice. This is especially true of such a rice having those advantages and being substantially free from typical parboiled flavor and color. Provision of a parboiled rice product having a full complement of advantageous features without one or more of the common shortcomings would be a substantial advance over the technology discussed heretofore.