what is acid value of oil ?
We often hear that acid value and peroxide value are used as indicators of the quality of oil products, but what do they mean in essence? "It may be unfair to judge the quality of oil only by the value of a single index. As long as we understand the scientific significance they represent, we will know why." The meaning of acid value is the milligram of potassium hydroxide (KOH) required to neutralize one gram of oil, which mainly represents the content of free fatty acids in oil. The allowable acid value standards of different kinds of oils and fats, pre refined or refined oils and fats vary greatly. Common olive oil products can be divided into virgin olive oil, refined olive oil and mixed olive oil. Virgin olive oil is divided into super grade and intermediate grade. The General Administration of quality supervision, inspection and Quarantine of the people's Republic of China stipulates that the acid value of refined olive oil must be less than 0.6mg KOH / g, while the acid value of virgin olive oil ranges from 1.6mg KOH / g for special grade products to 4.0mg KOH / g for intermediate virgin olive oil. If only the acid value of the product is used as the index, it will become that refined olive oil is better than virgin olive oil. Can it be compared in this way? In fact, these figures can only be interpreted as that acid price is one of the specifications of oil commodities. The quality of the product cannot be determined solely from the high or low price of edible oleic acid. "
Moreover, the peroxide value of oil alone can not reflect the quality of oil. The value of peroxide value represents the amount of peroxide in oil products, which is defined as the milliequivalent of peroxide per kilogram of oil. The oxidation process of oil belongs to the chain reaction of oxygen molecule intervention and free radical generation. The oxidation process will be accompanied by the production of highly reactive lipid peroxides. Because the lipid peroxides themselves are quite unstable, most of the processes of oil oxidation and deterioration will continue to crack, produce new free radical molecules or further produce aldehydes, ketones and ketones with low molecular weight Alcohol, acid and other oxides. Therefore, the change curve of peroxide value in the oxidation process first increases and then decreases with time. Looking at the peroxide value of oil alone can not reflect the current oxidation degree of oil. The quality of oil can not be reflected by a single index. Several quality indexes must be taken out and compared together to be accurate. The quality of oil can not be judged only by the acid value or peroxide value.
Factors affecting acid value
Edible oil is divided into pressed oil (such as peanut oil, sesame oil, etc.) and refined oil (such as soybean oil, sunflower oil, corn oil, etc.) according to whether it has been refined or not. If compared with the oil of the same oil source, the acid value of pressed oil is higher than that of refined oil. There are many factors affecting the acid value, and the raw materials for oil extraction are one of the important factors, such as the maturity and freshness of vegetable oil raw materials, as well as the processing methods such as baking, baking or moisture adjustment before oil extraction. The high acid value will increase the refining cost and reduce the oil yield for the crude oil that must be refined later. Generally speaking, too high acid value will affect the stability of oil, and the operation of oil refining is to remove the non triglycerides in crude oil (including free fatty acids, phospholipids, pigments, fat soluble vitamins, plant sterols and fat soluble phytochemicals in raw materials), and only retain the triglycerides in oil, in order to improve the stability of oil and facilitate preservation. Therefore, refined edible oil often has low acid value.
For consumers, although the acid value of refined edible oil products is low, the storage environment will still affect the oxidative cracking rate of oil. The process of transforming triglycerides into free fatty acids is a hydrolysis reaction. Some people will put edible oil in the refrigerator and take it out of the refrigerator when cooking. They will open the bottle cap before returning to temperature. In this way, the water molecules in the air will easily condense and come into contact with the oil, increasing the chance of oil hydrolysis and the storage temperature of oil products Whether the packaging container is protected from light, the area of the upper space in contact with the air, the length of opening time, etc. may affect the causes of oxidative rancidity. Without refined edible oil, unsaponifiable components in oil are also important factors affecting oxidative rancidity of oil.
An effective method to reduce acid value refining
In terms of refined edible oil, the main factor affecting the oxidation stability of oil is the fatty acid composition of oil itself. Triglyceride is the main component of edible oil, which is composed of one glycerol and three fatty acid molecules. Different edible oils have different fatty acid compositions. The chemical structure of fatty acids containing one double bond is called monounsaturated fatty acids, and those containing more than two double bonds are called polyunsaturated fatty acids. Generally speaking, the more polyunsaturated fatty acids in the fatty acid composition of edible oil, the easier oxidation and the worse oxidation stability.
In fact, the most effective way for any kind of oil to slow down deterioration and reduce acid value is to refine the oil and remove free fatty acids and other components that may affect oxidation stability. Free fatty acid is an unavoidable component in oil. In the past, when there was no oil refining, as long as there was no obvious smell of oil consumption, we would not care about whether the acid value was too high. Modern refining technology is mature, and free fatty acids can be removed to improve the stability of oil.
As far as raw oil has not been refined, what the acid value really affects is the manufacturer's production cost. The higher the acid value of crude oil, the lower the refining yield and the higher the cost. Therefore, most manufacturers will regard the acid value of crude oil as an acceptance specification. When crude oil raw materials enter the plant, if the acid price of fruit exceeds the acceptable index, there may be two follow-up practices, First, the manufacturer evaluates that the existing refining equipment is still capable of processing the products to meet the preset standards. At this time, the manufacturer will reduce the price to offset the loss of increased production cost and poor yield of finished products. Second, the batch of crude oil products that do not meet the acceptable specifications will not be returned.
