CEREAL TECHNOLOGY: RICE PROCESSING
Diagram
Rice steamed before milling, and rice milled as it is
Parboiled rice
Paddy soaked, steamed and dried before milling. Nutrients move into the grain, and the cooked rice is firm and separate.
↓
Raw rice
Paddy milled without any soaking or steaming. The grain is milled as it is, and the cooked rice is softer and stickier.
Figure 1: Parboiled rice is soaked, steamed and dried before milling, so it keeps more nutrients and cooks firmer than raw rice. Drawn in our own style from the rice production discussion in Owens, Cereals Processing Technology, chapter Rice production, p. 79, and in Aten, Equipment for the Processing of Rice, FAO, 1953, chapter Parboiling, p. 47.
Introduction
Parboiled rice and raw rice are both milled rice, but parboiled rice is treated before milling and raw rice is not. Parboiling means the paddy, the rice still in its husk, is soaked in water, steamed, and dried before it is milled. The steaming drives nutrients from the bran into the endosperm, and it gelatinises the starch, so the grain hardens and the cooked rice comes out firm and separate, with more nutrients than raw milled rice. Raw rice is paddy milled without any soaking or steaming, so its nutrients stay in the bran and are lost in milling, and the cooked rice is softer and stickier.
Quick comparison
Table 1: Parboiled Rice compared with Raw Rice
| Aspect | Parboiled Rice | Raw Rice |
|---|---|---|
| Treatment before milling | Soaked, steamed and dried | None, milled as it is |
| Nutrients | More kept, because steaming drives nutrients from the bran into the endosperm | Fewer kept, because nutrients in the bran are lost in milling |
| Starch | Gelatinised by steaming, so the grain hardens | Not gelatinised before milling |
| Grain hardness | Harder, so it breaks less in milling | Softer, so it breaks more in milling |
| Cooked texture | Firm and separate, the grains do not stick | Softer and stickier |
| Cooking time | Longer, because the grain is hard | Shorter |
| Colour | Slightly yellow from the steaming | White |
| Keeping quality | Keeps well, and resists insects better | Keeps well, but the grain is more fragile |
| Milling yield | Higher whole grain yield, because fewer grains break | Lower whole grain yield, because more grains break |
| Use | Rice dishes where firm, separate grains are wanted | Everyday rice dishes where soft rice is wanted |
Scientific explanation
Definition
Parboiled rice is rice made by soaking paddy in water, steaming it, and drying it before milling, so that nutrients move from the bran into the endosperm and the starch gelatinises. Raw rice is rice milled from paddy without any soaking, steaming or drying before milling.
Working principle
In parboiling, the paddy is soaked so water enters the grain, then steamed so the heat drives water-soluble nutrients, including B vitamins, from the bran inward into the endosperm, and gelatinises the starch. The grain is then dried and milled. Because the starch has gelatinised, the grain is harder, so fewer grains break in milling, and the cooked rice stays firm and separate. In raw rice, the paddy goes straight to milling, so the nutrients stay in the bran and are rubbed away with it, the starch is not gelatinised, and the cooked rice is softer and sticks together more.
Apparatus
The books used here name no laboratory apparatus for measuring or comparing either term, so no apparatus list is given.
Measurement procedure
The books used here describe parboiling in outline, not as a laboratory procedure with volumes and steps, so no procedure is invented here. In outline, paddy is soaked in water, steamed, dried, and then milled in the usual way. The FAO source for parboiling is Aten, Equipment for the Processing of Rice, 1953, which is old, and the newer source is Owens, Cereals Processing Technology, 2001.
Factors affecting readings
For parboiled rice, the soaking time, the steaming and the drying decide the result: too little steaming leaves the nutrients in the bran, and too much can darken the grain. For raw rice, the milling decides how much bran is removed and how many grains break. In both cases the variety decides the texture: long grain rice cooks separate, short grain rice cooks sticky.
Formulas and calculations
The books used here give no formula for either term, so no formula is given here.
Industrial applications
- Parboiled rice: rice dishes where firm, separate grains are wanted, and where more nutrients are wanted from milled rice.
- Parboiled rice: large scale cooking and canteens, where the firm grain survives holding and reheating.
- Parboiled rice: rice for milling in hot climates, where the hard grain breaks less and resists insects.
- Raw rice: everyday rice dishes where soft, mild rice is wanted.
- Raw rice: rice flour, noodles and breakfast foods made from milled rice.
Advantages and limitations
- Parboiled rice, advantage: it keeps more nutrients than raw milled rice, the grain breaks less, and the cooked rice stays firm and separate.
- Parboiled rice, limitation: it takes longer to cook, it has a slight yellow colour, and it needs the extra soaking, steaming and drying steps.
- Raw rice, advantage: it needs no treatment before milling, cooks fast, and gives soft white rice.
- Raw rice, limitation: milling removes the bran and its nutrients, the grain breaks more, and the cooked rice is softer and stickier.
FAQ
What is the difference between parboiled rice and raw rice?
Parboiled rice is soaked, steamed and dried before milling, while raw rice is milled without any treatment.
Why does parboiled rice keep more nutrients?
Steaming drives nutrients from the bran into the endosperm before milling, so they are not rubbed away with the bran.
Why is parboiled rice firmer?
Steaming gelatinises the starch and hardens the grain, so the cooked rice stays firm and separate.
Why is parboiled rice slightly yellow?
The steaming step gives the grain a slight yellow colour.
Is the FAO parboiling source old?
Yes. The FAO source is Aten, 1953, and it is used here with the newer source, Owens, Cereals Processing Technology, 2001.
References and further reading
- FTN books on disk: Cereal Technology.
- Owens, Gavin. Cereals Processing Technology. CRC Press, 2001, chapter Rice production, p. 79.
- Aten, A. Equipment for the Processing of Rice. FAO, 1953, chapter Parboiling, p. 47. Note: this source is old, and it is used here alongside Owens, 2001.
