High Costs Of The Isolation Biology Essay

Rice (Oryza sativa) is like all cereals a member of the grass family (Poaceae).

It is one of the world’s most consumed staple food and has been in use for more than 5000 years. (Bao and Bergman, 2004).

A large majority of the world’s rice is produced in Asia. With China, India, Indonesia and Bangladesh taking up to 70% of the world’s production.

When we look at the difference in rice types (see further) indica rice is by far the most produced type.

Structure of rice

When harvested, rice doesn’t look at all like the rice one could find in his meal. Harvested rice has a hull attached and is called "paddy rice". The hull constitutes about 20% of the weight of rough rice and is made of cellulose, lignin, arabinoxylans and ash. The rice hull is hard, non digestible and could damage the machines that are used in the further processing of rice. This means it has to be removed, the machine that is most used today for this process is the rubber-roll sheller. Rice hulls have high levels of lignin and silica which makes them of rather low value both nutritionally and commercially (even though lately rice hulls have been used as insulation material).

Once the hull has been removed, the obtained product is brown rice. Although brown rice is more nutritious than white rice, most of the sold rice is white rice.

That is why in a last step brown rice is milled. This milling removes the bran from the kernel to produce white rice. During this process some rice breaking inevitably occurs in the milling machines, which leads to "broken rice". It is this broken rice that is used in the starch refining process.

Figure 1 to 3 give an overview of the 3 different stadiums.

Paddy Rice_24.jpg

Figure : paddy rice


Figure : Brown rice


Figure : white rice

Figure 4 shows the structure of a rice kernel. On the outside is the hull which is made up of the floral envelopes, i.e., the lemma and palea. The next layer is the bran that is made up of the pericarp, the nucellus, the seed coat and the aleurone layer. And on the inside of the kernel is the endosperm. The aleurone layer is in fact the outermost layer of the endosperm but it is removed during the milling to produce white rice. The endosperm is made up of a subaleurone layer and starchy endosperm.

With oat, rice is the only cereal with compound starch granules (i.e., large granules made up of many small granules). The individual rice starch granules are small, averaging 2-4 µm.


Figure : rice kernel (http://www.fao.org/docrep/t0567e/T0567E01.GIF)


Origins of starch

Starch is in nature a way for plants to store their energy. During the photosynthesis process, carbon dioxide and water are transformed into transistory starch. This transistory starch is degraded shortly after production to sucrose and transported to seeds, where it is laid down as the plant’s main storage carbohydrate.

Starch is a carbohydrate build up of a large number of glucose molecules which are joined by glycosidic bonds. It is not only important for plants but is also an important source of energy for humans and is an important compound of the human diet. Starch is present in all kinds of things, but the most important sources of starch are cereals (rice, wheat and maize) and root vegetables (potatoes). In rice, starch is the most important component (>80% mass).

In rice and other cereals, starch granules are formed in amyloplasts. For rice, each amyloplast contains many starch granules. In case of energy demand, amyloplasts are able to transform starch into glucose.

Starch granules in rice are small (2-5µm) and are polygonal in shape. In the grain they occur as compound granules. The size of the rice starch granules is very small compared to other starch granules such as potatoe starch granules (up to 100µm). Other starches, like oat, also have small granules, but rice starch differs from those starches because the compound granules are still quite small.

Starch composition

Most types of cereal starches consist primarily (98%) of polymeric carbohydrate material, built up of α-D-glucopyranosyl. In the polymer the monomers are linked to the neighbouring glucose by either α-1,4 or α-1,6 bonds. While α-1,4 bonds give the polymer a linear structure the α-1,6 bonds result in a branched structure. The linear polymer is called amylose and the branched is amylopectin. Amylose and amylopectin have different structures but also different properties.

Amylose is relatively long and although it is generally assumed to be linear, it is today well established that some molecules (25-55%) are branched by α-1,6 bonds. The average chain length (CL) of amylose is 250-670 and the degree of polymerisation (DPn) is 800-4920. For rice starch the CL is 250-370 and the DPn is 920-1110, with a slight difference between indica en japonica rice. Its molecular weight is approximately 1x105-1x106. Amylose is insoluble in water and its long, linear structure gives it unique properties such as its ability to form complexes with iodine, organic alcohols or fatty acids.

Amylopectin is highly branched with only 5-6% α-1,6 bonds. Because of this high level of branching the average chain length (CL) of amylopectin is no more than 17-24, and of course on the other hand the degree of polymerisation is much higher than amylose with DPn going from 4700 to 12800.

This means for rice starch a CL of 19-23 and a DPn of 8200-12800.

The molecular weight is approximately 108, making it a very huge molecule, one of the largest in nature.

There are three types of chains in the amylopectin molecule. A, B and C chains. The A chains are composed of glucose linked α-1,4 and are the external chains; B chains are composed of glucose linked α-1,4 and α-1,6; and C chains are made up of glucose with α-1,4 and α-1,6 linkages. C chains are the only carrying a reducing end.

In most cereal starches the relative weight percentage of amylopectin range between 72 and 82%, for 18 to 33% of amylose. However, in rice some mutants have been found that contain less than 1% amylose. These mutants are called "waxy rices".

Besides amylose and amylopectin starch contains several minor constituents. The most important are lipids and proteins. Starch contains more or less 1% lipids and 0,25% proteins.

For rice the non-waxy starches contain 0,9-1,3% lipids whereas waxy starches contain negligible amounts of lipids. Another minor constituent is phosphorus. It is the most abundant mineral in starch with a concentration of 0-20 ppm in cereal starches.

Isolation of rice starch

High costs of the isolation

Although it is not a very complicated process, the isolation of starch out of rice is limited. This is because of its relative high cost. This high cost comes with the way the starch is isolated: through alkaline steeping.

Long soaking times and lots of water, to remove the residual sodium chloride from the rice, are just the first costs. At the end of the process, because of the large quantity of alkali and salt used, wastewater needs to be treated and this happens at a high cost.

Rice starch applications

The high cost means that rice starch needs to have some special properties that one couldn’t get from another, cheaper starch. And as a fact, rice starch has some very specific properties.

The colour is the first one, rice starch has a nice white colour. Then there is the size of the granules, as discussed earlier they are no bigger than 2-5µm. Besides a tiny size, granules also tend to have a very narrowed size distribution. This allows the granules to form a plain surface.

Another important property is the very neutral odour and taste of rice starch.

And at last, rice starch is known for causing minimal allergic reactions.

Because of this properties rice starch is widely used in many sectors and for many applications.

In the food industry rice starch is used in its native form as the major component of many foods. When modified, it can be used as an additive. Because of its small granular size, high water retention and low synergesis it can be used as a substitute for fat in milk based beverages, ice cream, yoghurt and many others. Other application are for example the coating of ‘tic tacs’ where the starches ability to form a plain surface is very useful, it is also used as an additive to meat to minimize rancidity during storage.

In the past Beneo Remy also used to produce rice starch-based glue. This glue was used for wallpaper glue or as glue to stick labels to beer bottles. It had the advantage to be easily removable with water.

The isolation process

The isolation of starch happens in a multistep process. Figure 5 gives an overview of the different steps of the production process.


Figure 5: Overview of the production process of rice starch

The isolation of starch starts with the soaking of the broken rice.

This step is important for two reasons. The first reason is that, as discussed before, starch granules have rice proteins tightly associated on their surface. Because high purity rice starch is wanted with low surface protein-lipid contamination, the proteins need to be removed. Glutelin, the major protein in rice, is alkali-soluble and leaches out during soaking.

The second reason for the soaking is that it weakens the rice kernel and prepares it for the wet-milling, this way it can be soaked without damaging the starch. In this process alkaline has an important role: making the broken rice porous and loosening and swelling the starchy endosperm.

Alkaline brings the hydrophobic glutelin in suspension and separates it from the starch.

The next step is wet milling of the rice, for this step it is important that the rice has been soaked. Milling of dry rice will engender more damaged starch. Some extra water is also ad during the milling to make sure a minimal amount of starch is damaged. Not much is known about soaking and milling of rice, but it is a possibility that after the soaking, within the rice kernel the proteins have dissolved into alkaline. They are in suspension but can’t get out of the kernel. After milling they become free. The physical result of wet milling is a white liquid that contains rice flour dissolved in water.

It is now ready for the next step, the sieving. In this step fibers that have come free during the milling are removed.

The free protein and the starch in this solution now need to be separated. This is achieved by centrifuging the liquid. Three layers are obtained after centrifuging, the lowest is the starch, the middle one are the proteins and water lies on top. Only the starch is kept, and it is then washed with water in order to remove the residual alkaline.

Now in a last step the starch can be dryed. Starch, a fine and white powder is the result.

It is clear that the soaking of the rice is a very important step in the isolation of starch. This project will focus on this step and investigate if there is a way to improve the process that is now used.

For this 3 parameters will be investigated, the ratio rice-soaking solution; the influence of dust onto the production of starch and the effect of stirring during the time of soaking.

Not only will there be looked at the amount of protein that has leached out and the amount of starch and damaged starch in the rice, but also what the effects of the discussed parameters are on the physical behaviour of the rice kernels after soaking.