Like me, do you get confused when you look at nutritional information on packages of cereal boxes, sandwich packs, canned food, etc? We get bombarded about Calories, but there is no consistency.
Why is it that on the nutritional information, you see units like Calories, kilocalories, joules & kilojoules? What are we meant to follow? What does calories mean? Well, let me see if I can simplify the matter. Here goes.
Just like miles or meters/kilometres are used as a unit to measure the distance, the unit that is used to measure energy is designated by the term calorie. There are two types of calories, one is the scientific term & the other is the nutritional term.
Let’s get the scientific one out of the way first so that we don’t have to worry about it. The scientist use the term calorie, written with a lowercase ‘c’, also sometimes called gram calorie (the symbol used is cal) to measure energy. Simply, this is the amount of energy that is needed to raise the temperature of 1gram of water by 1 degree Celsius.
Right, that done, now lets concentrate on the food calories we are interested in when we are trying to loose weight. Again, it is a unit of energy – i.e. a way of describing how much energy your body could get from eating or drinking food. Even when we are sedentary or sitting still or sleeping, we need energy to keep our body functioning. We need energy for our heart to keep beating, digest the food we eat, to grow, to repair any damage, to think, etc. The amount of energy we need varies from person to person, depending on our basal metabolic rate (BMR), gender, age and the level of activity. We obtain this energy from the food we eat and often it is termed as ‘fuel’.
In the past, the unit that was used to measure energy was Calories, the symbol used being Cal. This was written with upper case ‘C’ instead of the scientific term designated with small c. There is a link to these two terms in that the big Calorie is equal to 1000 small calories. To make things even more complex, another unit decided by the ‘International System of Units’ (or SI units) has superseded the big Calorie. This is used to try to standardise the unit so that everyone uses the same unit – but this doesn’t always happen and the old system is still used by many packaging companies.
The new International System of Units is known as joules and this really is what we should all be looking at. However, this is not where the story ends. Some foods carry a lot of energy and the number that you get in joules can be quite large. To make this number smaller, the joules are converted to kilojoules (symbol, kJ) and/or kilocalories (symbol kcal). Just like when we are driving, we record our distance in kilometres rather than meters to keep the number small. There are 1000 meters in 1 kilometre just as there are 1000 joules in 1 kilojoules. We are getting there! We can now link all these different units to see exactly what they mean.
So, when you look at the nutritional value of energy, concentrate on kilojoules (kJ) and try to ignore the others to make life simple. The following numbers shows how these are interlinked, expressed in roughly rounded figures:
4.2 kilojoules (kJ) = 4200 joules (J) = 1 Calorie (Cal) = 1 kilocalorie (kcal) = 1000 calories (cal).
Simply put, 1 Calorie (Cal) is equal to 4.2 kilojoules (kJ).
On average, a healthy relatively active female needs about 8400kJ (roughly 2000 Cal) per day and a male needs about 12,600kJ (roughly 3000 Cal) per day. This is if you are your ideal weight for your age and height. Obviously you will need far more energy if you are a keen exerciser. Athletes need huge amounts of energy to perform. For example, if you are a Tour de France cyclist, it is one of the most demanding events in the world, needing peak output for several hours at a time and for many hours. For this, a male top cyclist will need about 30,000 kJ of energy per day! Conversely, if you consume more energy than you need, then you will store the extra energy as fat and put on weight. On the other hand, if you consume less energy than you need, then you will use up your stored fat or from your muscles and so will loose weight.
OK, we eat food to provide us with energy, which is measured in kilojoules. This amount of energy will depend on the food we eat and how much of it we eat. Energy cannot be created or destroyed; it is only converted from one form to another. Different types of food provide different amount of energy, for example, carbohydrates (e.g. rice, pasta, and potatoes), proteins (e.g. meat, fish or pulses) and fats (e.g. butter, oil, animal fat, etc.). Some foods, like vegetables and fruits are low in energy (known as low energy), whereas fats and alcohol are high in energy (known as dense energy). We need all sorts of food for a healthy body but all must be eaten in moderation, especially energy dense foods like fat & alcohol and particularly if one is overweight, or even worse, obese.
Just to give you some idea, the following amounts of energy are found in different food in 1 gram of each.
|Food (1 gram)
||37 kJ (9Cal)
||29 kJ (7 Cal)
||16 kJ (4 Cal)
||17 kJ (4 Cal)
||13 kJ (3 Cal)
||0 kJ (0 Cal)
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, it’s 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 doesn’t 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 mother’s worldly wisdom I can pass this onto my own children. But don’t take my word for it, go and try it out yourself!