Chemical reactions in cooking rice
Posted on August 23rd, 2012

Dr Hector Perera     London

In chemical reactions bonds break then they recombine with other chemicals in different pattern to give other chemical compounds. In chemistry we say, breaking bonds need heat then making bonds give out heat, these are quite common in thermo-chemistry. Even cooking gas needs heat to start that is an endothermic reaction but once started it gives so much heat then the overall reaction is exothermic. When bonds break they are the exothermic reactions and endo thermic reactions are where heat is absorbed.

When we make “Sinhala ach charu”, you put all the ingredients and close the lid of the bottle or the pot so the chemicals are in a closed environment then after two or three days, what do you get is the real smell of the pickle due to formation of an ester. When you need to cook rice, you need to boil rice with water then after forming cooked rice, the smell changes due to the formation of a different chemical that is unforgettable smell of cooked rice. If rice got burnt then you get the smell of “dankuda”, I am sure you know what I mean? This is not uncommon if one cooks rice on firewood stoves because lack of control of the fire. I must admit some people are quite used to cook on firewood stoves, so they get it perfect.

In any food, cooking makes a chemical change. When cooking chicken, fish or any vegetable curry, you add a few spices including salt and some water. All spices have so many aromatic chemicals and the smell is due to some volatile chemicals for example cinnamon has 12 aromatic chemicals and cloves, curry leaves, coriander, chilli a lot more. The watery media helps the reactions, may be it acts as an electrolyte, I never know. Whether you add or didn’t add water, most food have a large percentage of water but in cooking rice one needs to add water then heat undergo convection currents in supplying heat to rice grains. As we heat the pot of rice on fire, from the bottom of the pot to the top of the pot there is a temperature gradient. If the lid was in a closed position then there would be a time that there is a uniform temperature throughout. Within a matter of minutes, it starts frothing and often it overflows. Then they open until it settles for a matter of minutes then put the lid back, this is an important state of cooking. I have noticed most people open the lid and check the water or check whether it is cooked or not. In my view if the lid was opened, then even temperature gradient falls, loses some heat and the top layer looses more heat. In some cases then they add some cold water, some times stir the rice. Then what happens? The cooked rice get mixed with partially cooked ones, then some times it cooks alright but often it goes wrong. To my surprise, the servants knew the science of cooking rice, they never knew all these chemical reactions but got it right, it amazes me.

The chemicals volatility depends on temperature, quicker you heat by applying more heat, faster they come out so for this reason some spices are added in the coarse form so the release of chemicals is slow. In the presence of heat, spices, meat or in rice some chemicals bonds tend to breaks while some chemical bonds holding the compounds together. Breaking those bonds makes those foods softer, then recombination with other chemicals make easier to chew, gives a palatable taste and easier to digest.

Further research studies

Some research studies have been done on cooking rice. The mechanism of cooking rice was investigated in this study. The rheological method using the parallel plate plastometer was adopted for measurement of the degree of cooking. The range of temperatures measured ran from 75″”…”150°C. Experimental results showed that cooking rate followed the equation of a first order chemical reaction. They designated the proportional constant as the cooking rate constant, but the slope of Arrhenius plots of the cooking rate constants changed around 110°C. The activation energy of cooking at temperatures below 110°C and above 110°C was about 19,000 and 8,800 cal/mol respectively. The influences of water soaking time before cooking were also studied. They concluded that the cooking process comprises two mechanisms; at temperatures below 110°C the cooking rate is limited by the reaction rate of rice components with water; and at temperatures above 110°C it is limited by the rate of diffusion of water through the cooked layer toward the interface of uncooked core where the reaction occurs. The reaction rate constant and the diffusion coefficient of water were calculated by assuming the core model or shell-type model. No wonder the servants; “kussi amma” reduced the temperature of cooking rice at a certain stage, just by sheer observation, how clever they are? Many science discoveries were made by observation, like the gravitational pull. How these servants did knew to increase then to reduce the temperature when the rice grains are changing to cooked state? They worked nearly whole day in the kitchen cooking; or can I say synthesised chemicals, to me actually it is a food chemistry laboratory. I am sure not many people looked at this hard work of these so called servants that way.

The Chemical Change

Rice contains about 90% starches, cooking is essentially the reaction of starch in water at elevated temperature. Then the starch granules absorb water and swell as heating continues. It said when a certain temperature call gelatinization temperature is reached; the cell wall of the rice granule breaks and the starch granules turns viscous or turn into amylase and amylospectin.
Rice also has protein and fibre. These are on the outer layer of the grain or on barn but unfortunately they are removed by milling. If one needs to eat nutritious rice the answer is, eat unpolished rice or brown rice. I cannot understand why we remove the nutritious part and feed animals and just eat pure carbohydrate part only.

When we eat unpolished rice, it gives a feeling of fullness but the polished rice burns away quickly making the people feel hungry so they eat more and more, you mean then supermarkets have more business? As I said before as Asians, we prefer to eat rice and curries than eating all fried Western diets because they are not so quite healthy. Who would argue that fried foods such fish and chips or chicken chips with salt, sauce and vinegar are healthier than rice and curries? Whenever possible try and cook some rice or any other home made food than depending on food from takeaways, restaurants or pre-packed food from the supermarkets because the home made foods are cleaner, healthier and cheaper.

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