The Chemist's Recipe I: Curry Rice

In this section, we will have a look the chemistry inside curry rice. We will have a brief description about the key chemicals inside this recipe as well the bioactivities of the molecule which has the function as nutrient.

The Art of Cookery (1747)
by Hannah Glasse

Before we discuss the chemical compounds on curry and rice, it is better to have a quick overview about its history. The term of curry was used in English long before arrival of traders to the subcontinent and the first curry recipe in Britain appeared in The Art of Cookery Made Plain and Easy by Hannah Glasse in 1747. However, the early recipes did not include hot spices because chili plants were not introduced to India until the 1500's.


The popularity of curry increased when Coronation chicken was used to commemorate the coronation of Queen Elizabeth II in 1953 and chicken tikka masala is often referred to as the country's national dish of the United Kingdom. Moreover, the origins of chicken tikka masala are fabled, with origins including Glasgow or Birmingham.

Since curry is a complicated dish and consists a lot of ingredient, so there are a lot of chemical compounds in it. Hence, in this section we only discuss about curcumin from turmeric, capsaicin from chili, endorphins as the brain response, zingerone from ginger, and starches from rice.

Turmeric plant (left) and curcumin (right)

Firstly, curcumin is the principle ingredient of turmeric and it is brightly coloured and is used as food additive (E100). The brightly coloured of curcumin is due to the highly conjugated double bond (π-bond) system. Besides that, curcumin is known for its anti-tumor, antioxidant, anti-amyolid, and anti-inflammatory properties. However, it has also been shown to be carcinogenic in rare cases.

Chili peppers (left) and capsaicin (right)

Besides curcumin, the spicy sensation in eating curry is associated with chili peppers and capsaicin is responsible for that sensation. Capsaicin is a severe irritant and it is suggested to be a chemical defence agent, but birds are unaffected (mammals are not). Besides that, it is often used in pepper sprays and it can cause severe burns. Furthermore, arthritis can be treated by capsaicin. Capsaicin can bind to the receptor VR1 which also responds to abrasive and heat damage

SHU of certain chili peppers in comparison with pure capsaicin

The chili heat is measured in Scoville Heat Units (SHU) and this measurement is the dilution required for a chili extract not to taste hot in a sugar-water solution. Moreover, it is suggested to drink milk (alcohol is also suggested) rather than water to reduce the spiciness of the curry due to low polarity of capsaicin.

One of the example of endorphins (beta-endorphins)

When capsaicins bind to VR1 receptor which responsible for heat damage and abrasive, the brain responds it by releasing endorphins and enkephalins. Endorphins and enkephalins are released from the brain in response to pain and other stimuli and both are peptides; repeating polymers made up of amino acids. Moreover, endorphins are larger than enkephalins, usually 17 to 27 amino acids in length. The effects of these molecules can lead to feelings of pain relief or pleasure, so it is possible to be drugs. However, there is one problem as oral drugs, which will be hydrolysed in acidic environment of stomach as well the cleavage by protease. Hence, the drugs is hydrolysed, so it will not give the effect and it is suggested to be used as an injection drugs.

Ginger (left) and conversion of gingerol into zingerone (right)

The next main ingredients in curry is ginger and it was introduced to Jamaica in 1525. The active component is a molecule called gingerol, but is converted to zingerone when it is cooked. Zingerone gives food the characteristic taste and smell of ginger.

The structure of rice grain (left) and golden rice (right)

Lastly, the curry rice will not be complete with rice. Rice is the staple crop for most of the world and 700 million metric tonnes are produced each year. 90% of rice content comprises of starch and there are 2 types of starch in rice, amylose and amylopectin. The composition of amylose and amylopectin is different in each rice; for example in long grain rice amylose is the main constituent, whereas short grain rice is rich in amylopectin. Moreover, the bran layer contains protein and fibre and cooking causes the starch to swell and break out from the cells. Nowadays, scientists are working on so-called golden rice which is genetically modified to produce beta-carotene, the precursor to vitamin A.

The importance of rice as a food means it must be efficiently produced, so to increase the efficiency the rice agrochemicals, pesticides, can be used. The global sales of rice pesticides was $2 404 000 000 in 2001.

The rice agrochemicals

In general, there are 3 main agrochemicals that is used in these days:

• Insecticides which kills insects and/or their eggs
• Fungicides which kills fungi or their spores
• Herbicides which kills unwanted plants

Despite the chemicals can increase the efficiency, the toxicity and environmental issues become the problems of these agrochemicals.

The helical structure of amylose

Back to the main constituent of rice, amylose is a polymer of repeating glucose units and it forms a helix structure to which potassium triiodide can bind. Hence, this forms the basis of starch/iodine test. In the other hands, amylopectin is also a polymer of glucose which is branched at about every 24 - 30 repeats, whereas amylose is linear.

Amylopectin (left) and amylose (right)