Which cooking oil should you use?

Keen cooks know you’ll need at least a couple of different cooking oils in the pantry. When you’re choosing an oil, you’ll want to consider the type of fats it contains, what you’ll use it for and how it’s processed.

Knowing your fats

Most oils are 100% fat but not all fats are created equal.

You’ve probably heard about saturated, trans, monounsaturated and polyunsaturated fatty acids (see “Cholesterol and fat facts”). Fats and oils are made up of these fatty acids – though one type of fatty acid usually dominates (see our Table).

Monounsaturated and polyunsaturated fats are the healthiest choice.

Plant-based oils, such as canola, grapeseed, olive and rice bran, mostly contain mono- and polyunsaturated fats and are lower in saturated fat. Coconut oil (about 90% saturated fat) and palm oil (about 50%) are exceptions. The latter is often used in takeaways and commercially baked goods.

Don’t be swayed by oils claiming to be cholesterol-free or containing “no cholesterol”. Cholesterol comes from animal products, so plant oils contain virtually none anyway.

Nickie Hursthouse, the Heart Foundation’s national nutrition adviser, said monounsaturated oils, such as olive, canola and rice bran, are good choices to use for cooking. Some polyunsaturated fats shouldn’t be heated to high temperatures so aren’t as safe for cooking with (see “Smoke point”).

Saturated fat makes up more than 50% of the fat in animal-based fats, such as beef dripping, butter and ghee (clarified butter), so they shouldn’t be used as your main cooking oil.

The World Health Organization recommends reducing saturated fats to less than 10% of total energy intake.

Types of fat in oils

Smoke point

Some oils shouldn’t be heated to a high temperature because they can smoke and lose flavour and nutritional quality. This is called the smoke point. All oils have a different smoke point and heat stability, which can be affected by oil quality.

Dr Laurence Eyres, chair of the Oils and Fats Group of the New Zealand Institute of Food Science and Technology, said a well-produced oil with low acidity will have a high smoke point, while a poor-quality oil with high acidity will smoke in the pan.

“This means not all olive oils, for example, will have the same smoke point.”

Dr Eyres said the fatty acid composition of the oil can also affect the smoke point and heat stability.

“Polyunsaturated fats like flaxseed and regular sunflower aren’t generally suitable for high heat cooking because they are unstable when heated to high temperatures and susceptible to oxidation. They’re best for cold dishes such as salad dressings and dips.”

Refined oils, such as canola, grapeseed and peanut, have high smoke points. Butter has a lower smoke point so suits light sautéing, rather than frying. Ghee can be heated to higher temperatures so it can be fried or roasted.

For everyday cooking or the barbie, a neutral-tasting oil that doesn’t mask the flavour of your food is best. Olive oil and canola are good choices.

For salads, pasta and stir-fries, you may want an oil with a distinctive flavour. Most cold-pressed oils (extra-virgin olive oil, almond and avocado) are good options. They’re also good drizzled on meat, fish and vegetables, or for dipping bread. Cold-pressed nut oils are best in cold dishes because heating can destroy their delicate flavours. Peanut and sesame oils are good in Asian dishes.

How they’re made

Oils can be extracted from seeds, fruit or nuts.

• Cold-pressed: The oil is extracted by mechanical pressing and there’s little or no heat used. After it’s pressed the oil is filtered. Cold-pressed oils usually have a darker colour, stronger natural flavour and are higher in antioxidants such as vitamin E and polyphenols.

• Expeller-expressed/hot-pressed: The oil is obtained by squeezing the seed, fruit or nut at high pressure. The high-pressure squeezing generates heat so these oils can’t be called cold-pressed. Expeller-expressed oils retain most of their flavour, aroma and colour.

• Refined: Most oils produced on a large scale, such as canola, soya bean and grapeseed, are refined using heat and chemicals. The oil is extracted using a solvent and then goes through bleaching, deodorising and distilling processes. Refined oils have less flavour, aroma and colour, as well as fewer antioxidants. These oils are usually cheaper and more stable at higher temperatures.

• Light or extra-light: The oil is refined so it’s light in colour and taste. Like other oils, light oil is 100 percent fat so isn’t “light” in fat or kilojoules.

• Extra-virgin: The highest grade of an oil. It’s made from the first pressing and has minimal processing to maintain the flavour and aroma. As a result, it’s the oil with the highest levels of antioxidants. You’re more likely to see the extra-virgin tag on olive oil – to make this claim olive oil must meet certain chemical criteria and be assessed as top grade by an expert panel.

Olive oil uses other descriptors such as virgin and pure. Virgin olive oil has minor imperfections and a higher acidity level. Pure is a mix of refined and virgin oil, resulting in a milder taste.

Ms Hursthouse said the degree of processing should influence your buying decision because an oil that has undergone the least amount of processing will have more nutrients.

“When polyunsaturated oils are refined, it’s also possible a small amount of oxidised lipids can form, which is bad for your health. This can vary even between different brands of the same type of oil,” she said.

Green credentials

There’s a growing appetite for greener food choices. However, there’s limited independent research comparing the environmental impacts of different plant-based oils.

A 2018 study published in the journal Science estimated the global variation of greenhouse gas emissions, land use and other environmental indicators of different food groups, including palm, soybean, olive, rapeseed and sunflower oil. Of the five oils, it concluded palm had the highest greenhouse gas emissions but the lowest land use, water use and run-off of nutrients into the environment.

However, palm oil has come under fire for the type of land the crop occupies. Palm oil plantations are a major driver of deforestation in Indonesia and Malaysia, destroying the habitat of endangered species such as the orangutan and rhino. Soybean oil plantations have also copped flak for fuelling the destruction of rainforests in South America.

To quell criticisms, the industry-led Roundtable on Sustainable Palm Oil and Roundtable on Responsible Soy Association were set up to certify “good” oil. However, both schemes have been criticised because of inadequate traceability and failure to guarantee the oil that ends up in products is sustainably produced.

What about coconut oil? In general, the environmental impact of coconut oil is low. But as demand for the oil has grown, there’s concern that coastal mangroves are being cleared for coconut crops. The largest producers of coconut are Indonesia, the Philippines and India.

Sarah McLaren, Professor in Life Cycle Management at Massey University, said products grown in New Zealand, such as olive, hemp seed and rapeseed, are produced without destroying tropical rainforests and delicate ecosystems.

Packaged foods with “vegetable oils” in the ingredients list contain a blend of oils. Food labelling laws don’t require the specific oils to be labelled so palm oil may turn up in these blends, increasing the saturated fat content.

“It would be good to see an eco-certification scheme for oils in New Zealand so consumers can be assured oils have been produced with the environment in mind,” she said.

Plant-based oils have the edge on animal fats. The 2019 EAT-Lancet report on sustainable diets recommended eating plant-based oils low in saturated fat as an alternative to animal fats. There’s wide agreement we need to cut back on animal-based foods to help tackle climate change and dairy products are a key offender. This is mainly due to their methane emissions, use of nitrogen-containing fertilisers and effects on our waterways. A 2017 review in the Journal of Cleaner Production showed that, globally, butter had a higher carbon footprint (9.25kg CO2e/kg) than other dairy products, such as yoghurt (1.31) and cheese (8.55).

Looking after your oils

Heat, light and air can affect oil quality. Here are our top tips for getting the best out of your oil.

• Green or dark bottles, or tins, provide better protection from light than clear bottles. Don’t buy oils that have been displayed in a shop window or under fluorescent lights.
• Store oil tightly stoppered in a cool, dark place.
• Buy the freshest oil possible. Look for a pressed-on date: best-before dates aren’t always a good indicator of quality because you don’t know how old the oil is.
• Don’t stockpile. Oils don’t improve with age so use any open oil within 12 months.

Cholesterol and fat facts

Cholesterol is a fatty substance that’s essential to help build the hormones and nerve cells your body needs. But too much cholesterol may thicken the walls of your blood vessels, increasing your risk of heart disease and stroke. Cholesterol levels are linked to your intake of fatty acids rather than cholesterol-containing foods, such as meat and eggs.

Cholesterol mainly gets around in the blood attached to LDLs (low-density lipoproteins) and HDLs (high-density lipoproteins). LDLs are the “bad” form. If you have high levels in your blood, it’s likely some will be deposited as fatty streaks on your artery walls, which increases your risk of heart disease. In contrast, HDLs help slow this process by carrying cholesterol back to the liver for processing.

Saturated fat raises the total cholesterol and levels of “bad” LDL (low-density lipoprotein) cholesterol in your blood. Saturated fats can also promote blood clotting, which can lead to heart attacks or stroke.

Trans fat raises total cholesterol and LDLs, and may also decrease your levels of “good” HDL (high-density lipoprotein) cholesterol. Cooking oils contain negligible trans fats but small amounts occur naturally in butter. The harmful trans fats are formed when liquid oils are hydrogenated (turned into solid fats) to make them more stable to use.

Polyunsaturated and monounsaturated fats are “good” fats – they help lower LDL cholesterol levels.

Omega fatty acids are polyunsaturated fats that are essential for health – our body can’t make them, so we need them in our diet. Omega-6s are common in the oils of seeds and grains, such as sunflower and corn oil. Flaxseed and hemp seed oils are good sources of omega-3.