Grease is the general term for oil and fat. Generally, what is liquid at room temperature is called oil, and what is solid is called fat. The source of fat can be animals or plants. Animal fats are generally solid at room temperature, have good shortening properties, and are more commonly used.
The amount of peroxide is generally quantified by measuring the amount of free iodine released by the reaction between peroxide and potassium iodide, expressed in mmol of iodine produced in 1KG of grease. Peroxide value is the most widely used method to evaluate the degree of oxidation of oils, because the quality assurance and control of oils mainly lies in detecting early oxidation. The quality indicators in the international recommended standards for edible vegetable oils and China’s national standards for all levels of edible vegetable oils stipulate that the peroxide value should not exceed 10mmol.
The peroxide produced in the early stage of oxidation of oil is further decomposed to form secondary products containing hydroxyl groups, such as aldehydes, ketones, etc. These substances are the main source of fat staleness and bad odor. The aggregation amount of hydroxyl groups in these secondary products is measured and expressed as hydroxyl value. The general method is to distill out the hydroxyl compound and convert it into a hydrazone, then measure the absorbance at 440nm and convert it into mmol. The Chinese national standard requires that the hydroxyl value (mmol) of ordinary edible vegetable oil should not exceed 20, and that of refined edible vegetable oil and salad oil. It is required not to exceed 10. At the same time, the hydroxyl value is also an important quality indicator for predicting the storage stability of oils and products.
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