What heat does to food
If you can cast your mind back to high school chemistry you will know that chemical reactions can be triggered or sped up by heating the reagents up, and this is no different with food. You can think of food as a complex matrix of chemicals, and a food processing facility is often nothing more than giant chemistry lab. Heating increases the reactivity of molecules in the food by giving the atoms more energy, which makes the bonds between them weaker. These weakened bonds make the molecules less stables, and allows them to easily react with other molecules. The most common reaction you see when foods are heated up is oxidation, where parts of the food react with the oxygen present in the air. This reaction renders most nutrients useless, and sometimes even toxic.
Cold pressing foods means that there is less heat involved (and often less pressure), which means that the food will under go less oxidation, and so is healthier. The most common example is cold pressed oils such as coconut oil or olive oil, which are often sold as cold pressed (but not always).
Foods which should be cold pressed or processed
Below are some foods which are still commonly heat processed today, but are particularly vulnerable to being oxidized, and so should be cold pressed.
Oils (coconut oil, seed oils, fish oils and olive oil)
These products are all made up of fatty acids, which are very vulnerable to oxidation and will readily react with oxygen in the air when heated. Omega-3 fatty acids (which are high in fish oils) are particularly susceptible to oxidative damage, which renders them nutritional useless and sometimes even toxic.
Oxidation can also occur when the oil is heated to high temperatures in cooking. This means it is important to ensure all the oils you cook with are cold pressed (i.e not eat extracted), and you only use heat stable oils for cooking such as coconut oil, rather than olive oil.
Milk undergoes pasteurization which uses extreme heat to kill pathogens. This high temperature can also cause the fats to oxidise, kill probiotics and denature proteins – rendering all these nutrients useless. In years gone by, pasteurization was needed for public safety to ensure no pathogens were present in milk. However, with modern testing technology and the regularity with which milk is tested for pathogens it is no longer needed.
High temperatures are used in the drying process to make whey protein, and this can cause proteins in the whey to denature. This means that the consumer will not get some of the additional benefits various proteins exert on the consumer such as immune support. Heat processing can also reduce the digestion and absorption of the protein. More information what makes healthy whey protein can be found here.
Raw chocolate is high in polyphenols which have a number of health benefits, including promoting probiotics in the gut, helping your immune system and protecting the body from oxidative stress. Conventional chocolate processing uses high temperatures which oxidize the polypheols, making them useless. Raw chocolate is actually very healthy as long as there is no sugar added to it.
Foods which can benefit from being heated
Whilst it might seem tempting to adopt a ‘raw food’ diet, some foods actually benefit from being heated to some degree.
Vegetable cells are surrounded by a tough cell wall made of cellulose, which we are unable to digest. This can make accessing the nutrients in vegetables difficult, but cooking vegetables weakens this cell wall which makes these nutrients easier to access. This means that lightly cooking vegetables will often make more nutrients available to be absorbed, however, cooking can also cause some nutrients to oxidise (in the same way mentioned above), and antioxidant nutrients such as vitamin C are particularly susceptible to this. This means it is important to only lightly cook vegetables.
Other than benefiting from the cell wall weakening, cooking gives additional benefits to tomatoes. Tomatoes are high in a nutrient called lycopene, which, when heated is converted into a much more absorbable (and equally beneficial) form. This means that the lycopene present in forms of processed tomatoes (such as tomato purée and dehydrated tomatoes) are much more bioavailable, although other nutrients may be destroyed.
Unlike the proteins found in milk, egg protein is more bioavailable when it is cooked compared to raw egg protein.
Cold pressing oils is common practice, and it means that the oil is not extracted under damaging conditions. This results in a healthier but pricier oil. In order to maintain the benefits from cold pressed oil, you should avoid cooking with unstable oils, such as olive oil, as these oils are much more susceptible to heat damage.
Cold pressing benefits don’t start and end with oils though, and other foods can benefit from being cold pressed too, such as chocolate, milk and whey protein.