Adulterated peanut oil becomes less transparent. Pour the oil quickly from the bottle into a cup and observe the oil froth. Pure peanut oil produces large oil froth with many small bubbles around it that do not dissipate easily. When adulterated with cottonseed oil or crude cotton oil, the oil froth may appear slightly greenish-yellow or brownish-black. Its smell will have a distinct cottonseed oil odor. For oil adulterated with starch substances like rice water or noodle water, adding a few drops of iodine solution will produce a purple or blue-black color. Alternatively, place it in a transparent cup for two days and observe; cloudy suspended matter will inevitably appear.
Adulterated sesame paste oil becomes darker in color; for example, it turns dark red when adulterated with cottonseed oil and deep yellow when adulterated with rapeseed oil. Furthermore, pure sesame paste oil itself does not produce oil froth, and any froth that appears when pouring dissipates very quickly. If the oil froth disappears slowly, it indicates adulteration. Another method is to dip a chopstick in the oil and let a single drop fall onto a calm water surface. Pure sesame paste oil forms a large, colorless, and transparent oil slick, while adulterated oil will form smaller, thicker oil slicks.
Adulteration with inedible oils, such as adding tung oil, castor oil, or mineral oil to edible oil, can cause food poisoning, with mild cases leading to illness and severe cases to death. Chemical methods are often used for inspection. For example, to test for tung oil, take 5 drops of the oil and place it in a test tube, add 5 drops of concentrated sulfuric acid, then heat it in 60°C warm water for 15 minutes. If a cloudy or flocculent substance appears, it can be determined that tung oil is present.