Commonly known as rancidity Rancid . Grease during storage due to oxidation and air and further decomposed into bad odor phenomenon. Mainly because of oil in unsaturated fatty acid auto oxidation, produce peroxides, and then degraded into a complex mixture of volatile aldehydes, ketones, acids. The general oil rancidity density decreases, Iodine value Reduce, acid value Increase。

Rancid Definition

 Oil rancidity Oil rancidity
Not edible oil rancidity. Fat is exposed to the air, after the light, heat, humidity and air, or by the action of microorganisms, can produce a peculiar smell gas, this effect is called Rancidity .

Rancid origin

The double bond in the fat of the unsaturated fatty acid was in the air oxygen The oxidation, generating a complex mixture of low molecular weight aldehydes and acids, and light and heat accelerated the oxidation process. The fat in high temperature, high humidity and poor ventilation conditions, can be caused by the action of microorganisms hydrolysis , produced fatty acid and glycerol That fatty acids can be further generated by microbial action ketone .

Rancid classification

Rancid hydrolytic rancidity

Under appropriate conditions, the oil and water reaction glycerol And fatty acid reaction called Hydrolysis reaction . This reaction is reversible for the first step. hydrolysis The production of glycerol ester two, then hydrolyzed to glycerol ester hydrolysis into glycerol, finally. No significant effect on fat hydrolysis food nutritional value.

Rancid Oxidative rancidity

 Oxidative rancidity Oxidative rancidity
The fish oil layer is placed in the air, change its quality, can find the changes of oil quality can be divided into 3 stages. The first stage oxygen quality change is very small, very few, called induction period (the initiation period); the second stage absorption of oxygen, significant quality deterioration, called the proliferation stage (propagation period); the third stage oxygen tends to slow or even stop, called termination.

Rancid Reason

1 water
Oil is a hydrophobic material, with little water. Press the oil with dry oil, its water content is little. But in the oil industry under the conditions of production, as the raw materials of water is too large, the device is not perfect or adverse operation and other reasons, often make the production of oil too much moisture. In addition, in the oil storage and transportation process, by the ingress of water, also can make the oil moisture. Oil water increased, not only makes the hydrolysis of fats to strengthen, free fatty acids Increase, will increase the enzyme activity, is conducive to microbial growth and breeding. Therefore, the excessive moisture content in oil, it is easy to hydrolyze oil rancidity. Generally, oil water content more than 0.2%, Hydrolysis It will strengthen the free fatty acid will increase. More moisture, hydrolysis The faster, the fat will quickly deteriorate, lose edible value. Thus, the moisture content in the oil is an important condition for safe storage of oil, but also cause Oil rancidity An important factor in the deterioration.
Fat, especially unrefined Crude oil And usually contains various impurities, such as protein, lecithin, wax, cake, and other end seed oil insoluble Foots Solid etc.. These impurities are hydrophilic substances, can absorb moisture, conducive to microbial growth and reproduction, can accelerate the oil rancidity, is very harmful to the safety of oil storage. Usually oil impurity content is over 0.2%, it is easy to cause the decomposition of grease rancidity, impurity content more, Oil hydrolysis Rancidity speed is faster, the easier it is to lose the value of edible oil rancidity deterioration. Therefore, many impurities of oil storage. In general, unrefined oil contains large amounts of phospholipids, phospholipid during storage can be decomposed Phosphatidic acid The oil quality, reduce, cause hydrolysis Deterioration; Mucin The oil opacity darken, but also conducive to microbial breeding, cause Oil rancidity ; soybean oil , Rice bran oil All contain wax, although its content is very few, but can promote oil turbidity, reduce the quality of cake at the end, Foots The seed coat, and other substances, conducive to microbial breeding, will accelerate oil rancidity. The long-term storage of oil, mixed with the impurity and its content is over 0.2%, must be removed, the content is below 0.2%, in order to maintain the stability of the oil storage, to ensure the safety of storage.
3 air
The oxygen in the air is caused by oil oxidation ( Auto oxidation The main factors). Grease contact air, the unsaturated fatty acids will be in the air oxygen The oxidation. Peroxide value and free fatty acids Continue to increase, and lower down into short Carbon chain Aldehydes and ketones, resulting in a special odor, lose edible value. There is a close relationship between oil oxidation speed and air contact surface area size, the length of time and oil composition. Usually the oil air contact surface area, a long time, it is easy to Oxidative rancidity On the other hand, loaded; closed containers or storage in an inert gas in oil, it can improve the storage stability, generally not easily oxidized.
4 temperature
Oil temperature can accelerate it oxidation reaction , enhance lipase The activity of microbial growth, promote breeding, and the secretion of protease, lipase, the unsaturated fatty acid in accelerating oil Oxidative decomposition Rancidity metamorphism. The higher the temperature is, the longer the time of high temperature, oil
 The unsaturated fatty acid structure The unsaturated fatty acid structure
Fat rancidity deterioration is faster (at 60 DEG -100 DEG range; average rise of 10 degrees celsius, Oil rancidity The speed can be doubled), rancidity process so as to reduce the temperature can suspend or delay oil, improve storage stability, ensure the safety of storage.
5 Sun
In the sunlight Ultraviolet That has high energy, can be activated oxygen, can make the grease Oxidative rancidity Deterioration。 Oil when exposed to sunlight, under the irradiation of ultraviolet light, often can form a small amount ozone . When the oil in Unsaturated fatty With ozone, can form a double bond in its place Ozonide . Ozone in water under the influence will be further decomposed into aldehyde and ketone compounds to produce oil Rancid taste Lost, edible value. At the same time, in the sunlight, contained in the oil Vitamin E Damage, antioxidant function, and thus will accelerate the rate of oil oxidation.

Rancid Matters needing attention

"Family doctor newspaper" reported that the oil is a kind of easily perishable food, mainly the deterioration is rancidity, fatty acid oxidation, can smell the odor (sensory Rancid taste Or moldy flavor). The study confirmed that eating fat rancidity will produce great harm to the human body, such as cancer. Many people love to eat vegetable oil, but you may not know, more easily than lard oil deterioration. The reason is that the oil which is rich in unsaturated fatty acid composition of the oil, and in Chemical structure Than Saturated fatty acid More instability.

Rancid Prevention

In order to prevent oil deterioration, feasible method:
1 buy the factory soon fresh Edible oil And other food, do not wait until close to the shelf life to eat.
2 of the purchase of nitrogen of vegetable oil in small packaging.
3 edible oil and food containing fat should be stored in low temperature place, avoid sunlight and heat the stove directly influence, so as to accelerate the Oil rancidity .
4 after opening the package of food should be eaten as soon as possible, for extra food should be stamped or sealed.

Rancid Food spoilage

Rancid introduce

Most foods contain nutrients such as protein, fat, in the face of these components oxygen By oxidation or by microbial decomposition or high temperature oxidation And the small molecule acid, and release the peculiar smell, leading to food Acid value Exceed the standard, rancidity phenomenon, serious when the volatile pungent Rancid taste .

Rancid cause

One The reason of the food itself Such as: processed food with oil of poor quality, improper storage environment, resulting in food contents appear very quickly, problems such as oxidative rancidity.
Two Food packaging reasons Packaging: unqualified easy to enter the air inside the packaging caused by oxidation and rancidity of food etc..
Food packaging reasons can be divided into three aspects
1) barrier such as poor oxygen barrier caused by poor packaging materials, packaging products in long-term storage process, the outside air slowly into the package due to oxidation of food ingredients; moreover, some high oil content products using composite poor effect composite membrane As the packaging materials, these materials, oil resistance is poor, easy to cause the composite film layer, resulting in decreased overall barrier packaging.
2) -- such as poor sealing Heat sealing Defective seal seal is poor, leading to the external air into the package from the heat sealing parts; in addition, some high temperature cooking product packaging is easy to appear in the high temperature steam sterilization process after the seal leakage, resulting in seal decline.
3) the physical and mechanical properties such as toughness, poor poor packaging materials, not resistant to rub or squeeze the contents of puncture or production process and storage process of the finished products, packaging in storage and transportation in the bag, bag breaking even pinhole, crease and not in time, lead to a large amount of oxygen and food contact oxidation metamorphism.