Another successful dinner party! A legacy left by my Dad, like him, I love entertaining friends and feeding them a good meal. Although I can cook many different cuisines, whenever friends are coming round for dinner, they always insist that they would like to eat my curry meal. During dinner, one of the consistently common questions from my friends is, ĎHow do you manage to cook your rice without it all sticking together? How do the grains stay separate and fluffy?í
Shh! If you promise not to tell everyone Iíll let you in on my secret. It is quite simple, but Iíd like to maintain the illusion of being very clever. It is a beautifully easy technique passed on from mother to daughter. I canít remember exactly when my mum told me, but it was her who instructed me to squeeze the juice of half a lemon and salt into a big pan of water and bringing it to the boil before adding the rice. Thatís all there is to it. When I was young, I never thought of questioning her as to why that made a difference, I just accepted my motherís pearls of wisdom.
However, my scientific mind got the better of me and I started questioning Ė how does it work? Well, it is all to do with Starch gelatinization Ė sounds like a big scary word, but all will be revealed. But I warn you; some parts are a bit complex to get your head round it.
Starch is a carbohydrate chain known as a polysaccharide. Starch is not soluble in water and forms a milky suspension when mixed in water. It is made of two molecules, amylose and amylopectin. Both are quite large molecules made of many smaller glucose molecules joined together. Starch is stored inside a granule, which in turn is contained within the grain of rice. The main difference between amylose and amylopectin is in their structure. Amylose is a long straight-chained molecule, a bit like a pearl necklace, whereas amylopectin is branched and spidery, as shown below:
Compared to other molecules they are huge, but you still need a microscope to see them. Let me explain a bit more. All living things are made of tiny cells that can only be seen under a microscope. Animals and plants are made of trillions of these cells. But within these cells are many chemical molecules, which are even smaller, if you can imagine that! Starch is one such molecule. Compared to starch, a water molecule is absolutely minuscule. Therefore starch, and in turn, amylose and amylopectin molecules, are relatively huge but very small compared to the size of the cell. These big molecules have to fit into a cramped space in the cell. In order to do this, the amylose chain twists into a helix, a coiled spring like structure as shown in the diagram above, to become much more compact. The branched amylopectin can then wrap itself around the coil, which means lots more molecules can be stored in the same amount of space. This coil and the wrapped spidery chains of amylose and amylopectin are held together by hydrogen bonds. Here is where things get more complex, so I will try to explain what hydrogen bonds are.
A hydrogen bond is an attraction between the positive and negative atoms held within the amylose and amylopectin where bridges are formed between say an oxygen and hydrogen atom in these molecules. It is a bit like holding hands to keep a group of people together! These hydrogen bonds are quite weak and can easily be broken but are very important in forming a stable molecule of, for example, starch to hold its structure together. So, hydrogen bonds form a stable starch molecule in the granule.
Now we come to the rice! When rice is being cooked, the starch molecules change. Water can penetrate the grain of rice and in turn the starch granule, which then swells into a roundish sack filled with a starch suspension. The granules can swell to about five times their normal size. When the water enters the granule, it weakens the hydrogen bonds holding the amylose and amylopectin together. The amylose is now released from the amylopecting and it sneaks out of the granule and the rice grain into the surrounding water. The water and heat breaks down some of the starch into smaller particles by breaking the chemical links between the glucose molecules of both amylose and amylopectin. So, instead of hundreds of glucose molecules joined together to make either amylose or amylopectin, they break into smaller glucose chains of 20, or 5 or 50, etc joined together. These smaller chains, which are released into the water, create a thick and viscous gel. Now the ordered state of starch becomes disordered. Imagine a group of children sitting in an ordered manner in the classroom, changing into complete disorder when let loose in the playground. It is this disordered state of starch that is known as gelatinization of starch.
As the water cools, the stiffness of the gel increases. It is this gel that escapes into the water and sticks to the grains of rice that makes the rice sticky. Hence, the more starch molecules escape into the water, the more viscous the water becomes.
Although a pain and undesireable in rice cooking, this process can be quite useful in catering for making a roux, forming moulds such as blancmange and using as thickeners and stabilizers in puddings, for which I have a particular weakness!
Now you know how rice becomes sticky, itís time to explain the role of the lemon juice! This gelatinization of starch can be made less viscous and is affected by some ingredients like acid, sugar and salt. This juice of lemon contains citric acid and so is acidic. You have to add the lemon juice to the boiling water before adding the rice, as once gelatinization has occurred the lemon juice will not work.
Lemon juice decreases the thickening power of the starch. The acid, or lemon juice, reduces the breakdown of hydrogen bonds, by making them more stable between the amylose & amylopectin chains. The acid also prevents the water from breaking down the starch into smaller particles. It stops the water breaking the links between the glucose molecules forming amylose and amylopectin.. Both these processes maintain the ordered structure of starch so that amylose is not released from amylopectin and this in turn stops it leaking out of the granule. The starch remains mostly in tact if fewer bonds are broken. This reduces the thickness of the water, which in turn stops the rice sticking.
The acidity of the lemon juice doesnít save all of the hydrogen bonds and some will still breakdown due to the hot water making the rice still potentially sticky, but less so compared to the rice without lemon juice. To stop further stickiness, I refresh or quickly rinse the rice in cold water after cooking and straining it in a colander. This also washes off any of the starch stuck to the surface of the rice.
To backtrack a bit, another trick is to wash the rice before cooking it in several changes of fresh water to remove as much surface starch as possible until the water runs clear and then soak it. If the rice is soaked for about 20 minutes or so, the rice grains will soften and will cook quicker too. This is basically the trick to cooking perfect, fluffy, non-sticky rice! Now that I have discovered the scientific secret to my motherís worldly wisdom I can pass this onto my own children. But donít take my word for it, go and try it out yourself!