rice production

Rice Processing – What Do You Need To Know?

Why is rice processing needed anyway? If we want the rice to be considered edible by humans, it must be “processed.” In fact, in its natural state, after harvesting and drying, it is covered by its “skin” (called husk). This husk has a natural composition similar to wood and, therefore not suitable for us to eat. The paddy rice collected in the paddy field is transformed into edible rice during the rice processing. The exact name of this process is husking.

Paddy Dryer and Rice Drying

Freshly harvested rice contains a certain amount of water, and this may depend on the more or less completed ripening, on the soaking of rainwater or dew, etc. The moisture content of the harvested product consistently exceeds 14%, which is considered the maximum limit allowed for appropriate preservation and storage and the suitable processing of the product intended for food.

You must consider the Paddy dryer an irreplaceable machine at the beginning of the rice processing cycle. There are different kinds of dryers on the market. Dryers differ between static ones, having a daily or prolonged process, and dynamic ones functioning with intermittent or continuous movement of the product.

Storage is also a critical factor. If it is well preserved, the rice in the warehouse continues to ripen, and the kernels become more consistent. The “seasoned” rice cook better than the freshly harvested. During the ripening phase at the warehouse (also called a resting stage), prolonged respiration occurs in the product.

However, storage shouldn’t take too long because aging makes starch and proteins less soluble in water. This results in a longer time required to cook rice well in the kitchen.

Rice Processing Steps

As mentioned before, the paddy rice harvested in the paddy field is rice in its raw state. As discussed above, it is dried to lower the humidity below 14% and is subsequently stored in special silos and warehouses waiting to be processed. 

rice processing plant

However, to obtain a perfect product, rice processing has several stages. The whole process is driven by industrial rice processing machines, which allow getting a high-quality final product.

So, how exactly does paddy processing go?

Proper rice processing is one of the most important variables determining quality of the end product. The first step is to remove the skin from the grain. The goal is to remove the outermost cell layers and the germ with minimal ruptures. The processing must provide a product with a pleasant appearance, having the best quality characteristics when cooked. Looking closely at a rice grain, we realize it is covered by a rough and hard shell: the husk, a fabric that must be removed to make the rice edible. The husk represents 20% of the weight of the grain. The oil contained in this surface layer gives brown rice its classic yellowish-brown color.

Rice Cleaning and Husking Process

Before removing the husk, the grain is separated from impurities, blades of grass, soil, and stones. At that point, the “husking” begins, which takes place by passing the paddy rice through the rice mill, that is, between two abrasive rollers that debark it. 

We find two rubber rollers of different speeds (tough and resistant rubber) inside the husking machine. The grain of rice passes through these two rollers, which are adjusted in such a way as to “crush” the grain only slightly and therefore not break it, and which have a different adjustment based on the variety of rice being worked. This slight crushing determines the husk (skin) separation from the whole grain (which we said we could also call husked).

So, refining or bleaching the pericarp – second layer or “rice bran” is eliminated. Even the “bleaching” occurs by abrasion, making the peeled rice pass between surfaces that remove the upper layer, a skin rich in proteins, vitamins, and mineral salts.

There are technologies to use removed rice bran to make rice bran oil out of it. But that’s already another topic.

  1. The first processing stage is called husking of the rice, allowing you to separate the grain from the husk. The current husking phase uses a rotating wheel and is a 90% efficient system. The remaining 10%, considered inedible at this stage, is separated from the brown rice by the paddy separator.

  2. The second phase allows the removal of the so-called green grain, the small and not yet ripe grains.

  3. The third phase is a stone remover machine that removes stones and pebbles from the rice. This occurs by separating elements of different specific weights, such as stones (heavier) and rice (lighter). This phase is critical because it allows us to avoid unpleasant problems such as breaking a tooth when we eat.

Sorting Rice by Dimensions and Color

The next phase of rice processing allows the removal of the rice germ, i.e., the broken rice with dimensions smaller than half of a grain. It means only the rice with dimensions equal to or greater than half of the grain remains. 

The rice obtained after the basic husking is not yet ready to be delivered to distribution.

In fact, it can present around 10% of aesthetic defects, consisting of red, green, or stained grains.

The optical sorting machine separates and rejects the finished product’s shades with non-compliant aesthetic characteristics.

The discarded percentage is passed through the optical sorter, and after a manufacturing process identical to the previous one, it is sent to the rice flour market.

So, after the husking, we should go directly to the selection of the grains to improve the quality and eliminate most of the defects. Broken grains, stained grains, grains that have not matured perfectly (i.e., green ones), red grains, straws, and impurities of various kinds should be removed from the final product.

How does an optical sorter work?

This selection and sorting of rice are made in several steps, both through sieves and sorting grids and with the aid of an optical sorter, where every single grain is “observed” and pushed very quickly through compressed air into different channels. 

An optical reader (adjustable) for each channel looks at everything that passes in front of it. When the reader “notices” something that has a different color from the one we want (in this case, brown or yellow), it gives an impulse to an ejector (blower), which pushes the grain, or the straw, or the impurity deemed unsuitable out of the circuit.

Important to notice that, especially on wholemeal, where the color difference between light brown wholemeal and the impurities, which can be yellow (straw or husk), green (unripe grains), or red, is not so easy to sort. So, it is considered normal to find some more defects compared to white rice, blemishes that do not affect the taste of the rice but only the aesthetic aspect.

Breakage separator and residues detector

The breaking of rice mostly depends on the type of rice. Long rice grains can break more easily than round rice. The grain can split into several parts at any particular point. The separator forwards the broken pieces to the grinding, aimed at rice flour production.

The rice processing machine’s last cycle phase is the debris detector passage. The control ensures that during transport or during/after the stone removal phase, there are no residues that do not comply with food regulations.

The machine features high sensitivity levels to detect the presence of any contaminant. This phase ends the processing: the rice is ready for consumption.

Rice Polishing Machine: How White Rice Is Made?

Bleaching is (and should be) a mechanical process. During the bleaching phase, the integral part of the rice (i.e., the bran) is removed by stone polishing. This phase takes place by pushing and pressing the grain of rice against a rough stone roller, trying not to push it too much so as not to break it. The strength of polishing also depends on the variety of rice processed.

Several rice polishing machines with different abrasive grains can be used for lighter and more gradual abrasion. This way, we can reduce the risk of breaking the rice grains. Anyway, in this case, we are talking about a mechanical operation. There is no chemical process in the rice processing phase, only mechanical.

After the bleaching step, the rice is bleached, meaning that the outer part of the grain was eliminated (about 10-30% of the weight), leaving the “heart” of the grain uncovered, which is made up of 80% starch, is white.

If we want to obtain semi-wholemeal rice, we must only remove part of the integral surface of the grain and not all of it. Then we go through fewer bleaching cycles, leaving the grain about half of the wholemeal amount (and therefore half oil, half vitamins, half mineral salts).

Also, once the bleaching or semi-bleaching is finished, it is necessary to move on to selecting the grains, as already described above, through sieves and trays and optical selection to eliminate defects based on the color.

Sometimes, however, the rice is only husked. Then we are talking about brown rice, which consumers much appreciate because it preserves the pericarp and bud of the rice grain and therefore has higher nutritional value than white rice. 

Rice Packaging

The last phase of rice processing is when rice is collected in storage and is ready for packaging. No preservatives or additives of any kind should be added to the final product. The rice is packaged in a protective gas: the oxygen is sucked out of the bag and replaced with nitrogen and carbon dioxide. This makes the storage times of the rice longer.

End Of Rice Processing

Once the processing is complete, white rice represents just over 60% of the original paddy rice. However, it should not be forgotten that not only the final appearance of the grain depends on the correct processing of paddy rice (as well as on its proper cultivation). How rice “behaves” in the kitchen also depends on that. 

Not perfectly ripe rice can be overcooked more quickly because the structure of the starch cells in the kernels is not perfect. The same happens if the rice has been poorly dried or badly preserved.

However, as mentioned above, the processing reduces some qualitative characteristics of the rice since it almost completely eliminates the cellular structures surrounding the endocarp; moreover, the elimination of the peripheral layers obtained with the processing produces a loss of the nutritional value of the rice. To avoid it, you can reduce the intensity of processing: in this case, the rice has a less shiny appearance and increases its consistency during cooking. 

Why Does Brown Rice Cook Longer?

Obviously, brown rice cooks longer because the pericarp limits the absorption of liquids, and over time it risks becoming rancid due to the high presence of fats. As mentioned before, it preserves pericarp and bud and therefore has higher percentages of nutrients than white rice.

brown rice cooking

Whether white, brown, or parboiled – the paddy rice that comes out of processing completely changes its appearance and is ready to be packaged and sold to the consumer.

Parboiled Rice Processing

Parboiled rice is the easiest to cook and store and is also the richest in nutrients. It is not a variety of rice but rather the result of special processing.

Parboiled rice is made using industrial machinery. However, it must be said that the process itself is very ancient and is based on the effect that the combination of water and heat exerts on the kernel. This is how parboiled rice is obtained in modern rice mills: the paddy is soaked in large tubs of hot water, then treated with high-temperature steam under pressure, and finally dried quickly.

The first consequence of these processes is that the mineral salts migrate towards the gelatinized starch core, where they will remain even after refining: this is why this rice is the richest in terms of nutrition. However, the parboiling process mostly modifies the grain’s structure, reducing its capacity to absorb liquids. The change is visible to the naked eye: the parboiled grain is translucent and amber. This rice can be cooked longer without disintegrating, so it is widely used in restaurants: it cooks quickly and can be cooked in advance and stored in the fridge without losing its characteristics.