Though home-pressed oil has more nutrients and flavor, it has some drawbacks as well. Photo: IC

Back in 2010, it was the reports of "gutter oil" - restaurants cooking with recycled waste oil - that disgusted the public. More recently, scaremongering posts on Sina Weibo have wondered whether oil made from genetically modified (GM) soybeans could cause cross-species infection. According to a Beijing Youth Daily report, over 90 percent of China's edible oil is made from GM soybeans. Given the unknowns about this ubiquitous foodstuff, some Beijingers have decided to play it safe by pressing their own cooking oil at home.

Yi Mu, 27, has been extracting oil at home since 2011. When asked why, she replied: "First of all, it is safe. Secondly, home-pressed oil has a different fragrance, which is actually its natural flavor. You can smell and taste the difference; when oil is extracted via industrial procedures and doctored with additives, the natural flavor gets lost." Nowadays, Yi and her family rarely cook with store-bought oil. "Since food safety is such a serious problem, we now only trust the oil we extract at home."

An oil press tends to be pricier than other common kitchen appliances. A search for "oil press" on the e-commerce website taobao.com turns up thousands of results, with prices ranging from 680 yuan ($110) to around 6,000 yuan. The most popular version sells for 1,780 yuan - 625 machines have been purchased in the past month.

Oil can be extracted from a wide variety of ingredients, including peanuts, walnuts, sesame seeds, rapeseeds, sunflower seeds, and camellia seeds. Generally speaking, 2.5 kilograms of raw material produces about one liter of oil. The ingredient should first be heated to make the oil easier to squeeze out - though with more advanced machines, the heating occurs directly inside the machine. It takes an hour to produce one liter of oil. Then one or two hours later, when the impurities settle, the oil is ready to be consumed or used for cooking.

According to nutritionist Yang Wenjiao, home-pressed oil does contain more nutrients - phytosterol, Vitamin E, phospholipids, isoflavones, and carotene- than processed oil. However, if the oil is not refined properly, it can contain harmful ingredients.

"Unesterified fatty acids make it easy for home-extracted oil to go rancid. Benzopyrene, acrylic aldehyde and aflatoxin are carcinogenic substances," said Yang. "That last one is produced by mildew. It's difficult to detect with the naked eye; even if you pick out the moldy beans, the toxin may still be present." Also, because the smoke point of home-made oil is lower, more inhalable particles will be produced when the oil is heated, which can cause harm to the respiratory system.

Yi admits there are drawbacks to home-extracted cooking oil. "It's true that the smoke is heavier, but our kitchen ventilator is stronger than the smoke," she joked. But she balks at the idea that it could be harmful to the health. "I do not understand how home-extracted oil could contain carcinogenic substances, since eating peanuts and walnuts will not cause cancer, and the machine only crushes the ingredients."

Yang said that the sellers of such machines may be overstating the nutritional benefits of home-pressed oil. She told Metropolitan, "I think it's OK to buy oil from the supermarket, but you need to be careful when choosing." Yang recommends that people who buy processed oil examine the color carefully; it should be neither too pale or too dark. However, it is impossible for consumers to tell by appearance whether oil is made from GM soybeans. The only way to know is by reading the label carefully. If it does not specify "non-genetically modified," then you should assume that it is.