Are you choosing your cooking oil wisely?

Mandy Lau, HK BioTek Nutritionist

When it comes to fat & oil, people tend to have strong association with diseases, and they are trying their very best to lower the intake of oil when possible. Being one of the most indispensable items in kitchen, how to choose cooking oil becomes an important issue. People are encouraged to use vegetable oils for cooking. As there are dozen types of cooking oil available in the market, are you choosing them wisely? Which oil suits your use?

Vegetable oil is regarded a healthier cooking oil because its saturated fat content is lower than that of animal fats. Over consumption of saturated fats is known to link with increased risk of cardiovascular disease. Nevertheless, the proportion of saturated fat alone in cooking oil is not enough to judge its healthiness. There are two major factors that determine the quality of cooking oil: the extraction method and the oxidative stability.

The first factor: the extraction method

In practice, there are two types of oil extraction: physical extraction and chemical extraction. Both mechanical pressing and cold pressing are physical methods. The basic principle is to use physical and mechanical extrusion to extract the fat from food ingredients. The raw materials are fried and steamed before processing in ordinary mechanical pressing. But in cold pressing, the raw materials will not be heated before pressing. The entire process will be controlled at 50‎°C or below. Physical extraction method is suitable for oil-rich seeds and fruits (having at least 50% of fat), such as coconut, olive, avocado, sesame and so on. Since no heating process is involved in cold pressing, the product retains the natural flavor and nutrients of the raw materials. Therefore, cold pressed oil is generally purer and healthier. Although cold pressing process is simple, the market price is quite high because of the low oil output rate and high cost.

As for general household vegetable oils, such as soybean oil, corn oil, canola oil, etc., because the fat content for their raw materials is not rich, chemical solvents are used to extract the oil content from these plants. Chemical oil extraction yields up to 97-99%, therefore it is a simple and cheap method of oil production. In simple terms, industrially made edible oil involves the following steps:

1. Chemical extraction: After crushing and heating the raw materials, add chemical solvent (such as ethane) to extract the raw oil

2. Solvent separation: The solvent is separated by a distillation system to form the semi-finished oil.

3. Refinement: The semi-finished oil is processed by degumming, deacidification, bleaching, and deodorization to become a clear, odourless cooking oil

Many people concern that the solvent is not completed recovered in the process of solvent separation, leading to residual solvent in the commercial oil. Indeed, any solvent residues in oil could lead to health issues. Long-term exposure to ethane can cause headaches, dizziness, muscle weakness, nausea, and even damage to the nervous system. Therefore, in response to the harmfulness of ethane residues, different organizations have set a maximum residue limit for ethane in cooking oil. Taking the European Union as an example, it has stipulated that the ethane residue in vegetable oil must not exceed 1 mg/kg.

To determine the quality of cooking oil, the extraction method is very important. The simpler the method of extraction, the lower the possibility of having chemical residues in cooking oil, and the more the nutritional value and antioxidants of the raw materials can be retained. The public should try to avoid chemically extract and over-refined cooking oil.

The second factor: the stability

Many people only consider the smoke point when they choose cooking oil. They think that the higher the smoke point, the healthier the choice. However, the smoking point should not be treated as the only consideration, because it does not reflect the stability of cooking oil.

The main component of cooking oil is triglyceride (composed of glycerol and three fatty acids). When the cooking temperature reaches the smoke point, triglyceride quickly decomposes into glycerol and free fatty acids, and it becomes the white smoke we see in the air. When the oil temperature keeps rising, the faster the decomposition speed, more and more glycerol are converted into acrolein (which is a carcinogen), so it explains why it is generally recommended to choose cooking oil with a high smoke point to avoid excessive generation of carcinogens at high temperatures.

By just considering the smoking point is far not enough, a good cooking oil needs to be stable. Fatty acids can be divided into saturated, monounsaturated and polyunsaturated. Unsaturated fatty acids (especially polyunsaturated ones) can easily be oxidized in high temperatures, and oxidized fatty acids can increase blood pressure, block blood vessels, leading to increased risk of cardiovascular disease. As spoiled cooking oil breaks down into polar compounds such as aldehydes, ketones, and acids, increasing the risk of cancer and neurodegenerative diseases as well. Organizations around the world list fried cooking oil as highly dangerous when the total polar compounds are higher than 24-27%. Oils with a higher proportion of polyunsaturated fatty acids tend to have lower stability and thus they cannot be used in high-temperature cooking. The following chart briefly lists the fatty acid ratios of common cooking oils:

Source: POS Pilot Plant Corporation

In 2018, Australian scholars studied the chemical stability of 10 common cooking oil, which include extra virgin olive oil, virgin olive oil, olive oil, canola oil, rice bran oil, grape seed oil, coconut oil, high oleic peanut oil, sunflower oil and avocado oil. Two experiments were conducted: heating the sample to 240°C and heating the sample continuously at 180°C for six hours. The purpose was to find out how heating temperature and time affect the stability of cooking oil. The proportion of polar compounds in the 10 cooking oils before the study was about 5.54-9.63%. It was found that after high temperature and prolonged heating, the extra virgin olive oil (8.47%) and coconut oil (9.30%) had the lowest polar compounds, while canola oil (22.43%), grape seed oil (19.79%), and Sunflower oil (15.57%) has the highest percentage of polar compounds at the end of the study. It is suggested that vegetable oils containing high levels of polyunsaturated fatty acids are not suitable for high temperature and prolonged cooking.

Different uses, different oils

By learning about the characteristics of each oil, you can maximise their benefits. The following table briefly summarizes the cooking oils recommended for different cooking methods:

Safety should be taken into account when considering your cooking oil. Even the oil contains lower saturated fat (healthier in general), if it is used improperly (such as cooking at high temperature), the carcinogens produced will have more significant impact on the body. Readers should carefully choose cooking oil according to uses, and also pay attention to production methods. No matter what kind of cooking oil you choose, low-temperature and low-oil cooking methods is preferred to avoid the loss of nutrients in the food and to reduce the possibility of cooking oil deterioration. Under no circumstances should cooking oil be reused, especially after deep-frying.

Reference:

Guillaume C., et al. “Evaluation of Chemical and Physical Changes in Different Commercial Oils during Heating”. Acta Scientific Nutritional Health 2.6 (2018): 02-11.