In the recent years, with increasing research in the field of nutrition, it is a given fact that eating healthy nutritious food will help improve health and energy levels. With this fact it should also be known that the process of cooking or heat transfer also plays a major role on the nutrients in food.
What is heat transfer?
Heat transfer can be explained as the transfer of thermal or heat energy from one body to another which depends upon the temperature and medium through which energy is being transferred. In cooking, heat transfer can be done by various methods such as stove, oven, microwave or fryer.
Heat transfer is a very important process of cooking food. It helps by potentially destroying the harmful bacteria and other microorganisms, which makes food healthy and easy to digest.
Some frequently used methods of heat transfer are –
- Conduction – this type of heating happens with direct contact. For example – a vessel on the stove will get heated from the bottom directly. The heat is then transferred to the food directly from molecule to molecule.
Conduction heat is also responsible for the heat energy to travel from the outside of food to the inside of it. Hence, conduction is also involved in convection and radiation processes.
- Convection – this combines conduction heat transfer and circulation of the heated molecules in the air to move away from the object to cooler areas. As these heated molecules move away, other cooler molecules are replaced by them. There are two ways of convection:
- Natural Convection – this is when the heated molecules at the bottom of a vessel are heated and rise above and cooler molecules take their place. This creates a circulating effect and distributes the heat evenly, in turn cooking food.
- Mechanical Convection – this is when the heated molecules are moved by an outside force. This shortens cooking time and also cooks food more evenly. For example – using fans or exhaust systems in ovens to move the hot air above and around food before throwing it out.
- Radiation – this is the process where heat and light waves strike and penetrate inside the food making it hot. There is no direct contact between the heat source and the food that has to be cooked. There are two main types of radiation:
- Infrared radiation – this usually involves the heating of an electric or ceramic heating element that give out electromagnetic waves, these waves travel in all directions at the speed of light and strike on the outer surface of the food that has to be prepared.
- Microwave radiation – this utilizes short, high frequency waves that penetrate inside food, making the water molecules present in them to create friction and transfer heat. While heating solid food, heat is transferred through conduction and while heating liquids convection process is used.
Heating And Its Effects On Nutrients
We have all seen the changes that occur to food while cooking. Like red meat turns brown, some liquids become solids, flavour changes, etc. have we ever wondered why? Well, here are some answered on how each nutrient reacts to heat.
- Proteins – Coagulation
Proteins that are obtained from plants and animals are made up of long chain amino acids. When heated, these break down to short chains and lose moisture. This makes them change forms from liquid or semi liquid to solid called coagulation.
Example – hard boiled or fried eggs.
Temperature of change – 140 degrees Fahrenheit
- Starch – Gelatinization
When starches are heated, they absorb the moisture around them making solid starchy foods softer. Starches can be added to soups or stews for thickening purposes. This process is called Gelatinization.
Example – rice getting larger and softer after boiling or adding flour to thicken soup.
Temperature of change – 150 degrees Fahrenheit
- Sugar – Caramelisation
Sugars tend to turn brown when heated, this is called caramelisation. This applies to all kinds of sugars, the naturally occurring ones too. This process can be associated with almost all the flavours we associate with cooking. This happens at a higher temperature than water also explaining the browning of foods only when cooked without water.
Example – Browning of bread on baking
Temperature of change – 338 degrees Fahrenheit
- Water – Evaporation
This process is something we all are familiar with. When water is heated, the molecules move faster and faster eventually turning into gas and evaporate. Since the water content is lost during cooking – it explains the solidification of foods on heating.
Example – spinach losing shape on heating
Temperature of change – 212 degrees Fahrenheit
- Fats – Melt
Unlike water, fats do not evaporate when heated but they do melt. At room temperature they can be solid, liquid or somewhere in between, but on applying heat they all turn into liquid. Since these foods burn at a very high temperature, they are preferred to be used as a medium to cook food rather than just an ingredient.
Examples – using butter or ghee to pan fry.
Temperature of change – varies depending on different fats.
Although cooking food can ease digestion and increase absorption, several key nutrients are also lost in the process of heating food. Such as –
- Water soluble vitamins – Vitamin C and B vitamins – thiamin (B1), riboflavin (B2), niacin (B3), pantothenic acid (B5), pyridoxine (B6), folic acid (B7) and cobalamin (B8).
- Fat soluble vitamins – Vitamins A, D, E and K.
- Minerals – mainly potassium, magnesium, sodium and calcium.
There are many ways to prevent the loss of nutrients from food without sacrificing the taste and other qualities.
It is important to select the right kind of cooking method for the right food to maximise the nutritional quality.
In general, cooking at low temperatures with minimal amount of water is the best way to retain nutrients in foods.
Do not let your nutrients be washed and go down the drain. Cook mindfully!