Analysis of causes of changes in acid value and peroxide value of fried foods and control measures

Understanding of related concepts

1. Composition of grease

Grease is a mixture of fatty acid triglycerides, composed of three elements: C, H, and O. The molecular composition is composed of one molecule of glycerol and three molecules of fatty acids. Different fatty acids determine the differences in oil properties.

2. Important indicators for measuring oil quality - acid value and peroxide value

    Acid value (AV) reflects the content of acidic substances (free fatty acids) in oils and fats (generally calculated using potassium hydroxide titration or automatic potentiometric titration, refer to GB 5009.229-2016 "National Food Safety Standard Determination of Acid Value in Foods". Peroxide value (POV) reflects the level of peroxide content in oils and fats (generally titrated and calculated using standard solutions of potassium iodide and sodium thiosulfate, refer to GB 5009.227-2016 "National Food Safety Standard Determination of Peroxide Value in Foods".

3. Antioxidant properties of frying oil

The antioxidant capacity of different oils is different, mainly because the types and contents of antioxidant components in oils are different, such as: alkenes, tocopherols, tocotrienols, BHA, BHT, TBHQ and other natural or synthetic antioxidants . When oil oxidizes, it first reacts with the antioxidants in the oil, and then reacts with the oil itself after the antioxidant components are consumed. The higher the unsaturated fatty acid content in oil, the easier it is to be oxidized. For example: soybean oil has a high content of unsaturated fatty acids (more than 85%) and has poor antioxidant properties; palm oil contains half and half saturated fatty acids and unsaturated fatty acids, and contains natural antioxidants such as tocopherols and tocotrienols, and has poor antioxidant properties. powerful. Active oxidation value AOM is an important indicator to measure the stability of oil. The higher the AOM value, the stronger the antioxidant capacity and the better the stability.

Analysis of the causes of rancidity of oils and fats

1. Two important processes of oil rancidity

The first part is the hydrolysis process of oil, that is, oil is directly hydrolyzed into glycerol and free fatty acids. In this process, the peroxide value does not change significantly, but the acid value will increase. The rancidity of this part is expressed by the acid value (under the premise that the moisture content in the oil is high Significant hydrolysis, one of the main sources of acidity in fried foods). The second part is that the double bonds of unsaturated fatty acids in the oil are oxidized and opened to form peroxides. At this time, the rancidity of this part is expressed by the peroxide value. The peroxide further reacts to generate lower-level aldehydes, ketones, acids and other compounds. At this time, the peroxide value will decrease and the acid value will increase. The rancidity of this part is expressed by the acid value (usually only in the case of severe deterioration). It will appear that under normal food storage conditions and shelf life, the acid value is generally relatively stable and not too sensitive to changes, and the acid value will not rise significantly). Generally, the increase in peroxide value will lead to an increase in acid value in the later stage, and acid value and peroxide value can coexist at the same time. Sometimes if the oil is unsaturated fatty acid, the peroxide value will appear first and then the acid value. The acid value will keep rising, while the peroxide value will rise and fall (in the case of severe deterioration).

2. Why does rancidity produce unfriendly odor?

On the one hand, it comes from the short-chain fatty acids (C4~C10) produced by hydrolysis. On the other hand, the carbon-carbon double bonds in the oil are oxidized into peroxides by oxygen, water or microorganisms in the air. The peroxides continue to decompose to produce Some low-molecular aldehydes, ketones or carboxylic acids with special odors lead to the production of odor.

3. Why do the acid prices of frying oil and finished products rise rapidly when frying green beans and orchid beans?

The increase in acid value is related to the free fatty acids produced by hydrolysis of oils and the acidic substances produced by the oxidation of unsaturated fatty acids. The acid value rises rapidly during the frying process, mainly due to the hydrolysis of fat. The higher the moisture content of fried food, the faster the oil will hydrolyze. Green beans and orchid beans generally need to be soaked for a long time before frying and have a high moisture content. The fatty acids produced by the hydrolysis of oil cause the oil to become acidic (after testing, oil is generally acidic during frying, with a pH value between 4 and 7). Acidic conditions are more conducive to the hydrolysis of oil. The fatty acids produced during hydrolysis cause the acid value to rise rapidly. main reason.

Change trends of acid value and peroxide value in food

Acidic substances are mainly free fatty acids produced by hydrolysis and oxidation. The acidity is mainly caused by the carboxyl group in the compound. The carboxyl group is relatively stable and is generally not easy to continue to be oxidized. Therefore, acidic substances generally do not change after they are generated. That is, the acid value in food will continue to rise, but during the normal food shelf life, the acid value is generally relatively stable. It will only change significantly under long-term or harsh storage conditions, so the acid value of the product is generally controlled before leaving the factory. The acid price will generally not exceed the standard during the shelf life. The peroxide value of vegetable oils is generally around 0.25g/100g, which is an inflection point or mutation point (so the national standard is limited to 0.25). After that, the oil quality will enter a period of rapid deterioration. Without completely blocking oxygen, the peroxide value of fried foods changes significantly during the shelf life due to the influence of light, temperature, metal ions in the product, etc. Before the change inflection point, the peroxide value rises gently (the higher the antioxidant properties of oils, the smaller the change). After the change inflection point, the peroxide value will rise significantly.

How to control the acid value and peroxide value of fried foods

Aiming at the causes of oil rancidity and controlling the conditions required for oil hydrolysis and oxidation, we can extend the shelf life of products and improve the quality of fried foods.

1) Drain as much water as possible from food raw materials before frying to reduce the moisture content of oil;

2) Keep the grease clean to avoid grease deterioration and taste changes caused by repeated frying of residues for a long time. The frying oil must be filtered after each shift (multi-stage filtration can be used);

3) Control the appropriate frying temperature and frying time while ensuring the process requirements;

4) It is best to have an oil-draining process for fried foods to reduce the impact of frying oil on the product itself (the quality of oil will significantly decrease after long-term frying, and the acid value, peroxide value and polarity will decrease significantly) molecules will rise significantly);

5) Strengthen the detection of the acid value and peroxide value of frying oil during the production process, and adjust the production oil in a timely manner. The operational limits for frying oil indicators should be set stricter than the national standard requirements (it is recommended that the acid value of frying oil should be controlled at 3.0mg/g and the peroxide value should be controlled within 0.15g/100g);

6) Use a more stable frying-resistant oil, such as edible palm oil instead of soybean oil (which will help control the increase in peroxide value);

7) Reasonable use of antioxidants (helps to control the increase in peroxide value and has no significant impact on the increase in acid value caused by hydrolysis);

8) The residual amount of oxygen in the finished product packaging and the oxygen barrier performance of the packaging are important factors that affect the peroxide value of food during the shelf life (the peroxide value increases by 0.1 per 100 grams of food, and approximately 8.8 mL of oxygen is consumed). Fried food is best packaged by vacuuming or adding a suitable deoxidizer. PE packaging bags cannot be used (the barrier performance is too poor)