The free fatty acids in vegetable oil are titrated with a potassium hydroxide standard solution, and the number of milligrams of potassium hydroxide consumed per gram of vegetable oil is called the acid value. Determination method Determination of acid value and acid value is recommended in GB/T5530-2005. The national standard can be found on the food partner network. During the processing, storage and circulation of fats and oils, under the action of heat, moisture, metals and microorganisms or enzymes, the oil will be hydrolyzed to release free fatty acids. The increase of free fatty acids in the oil will cause oil smoke spots and surface tension. Decreased, resulting in decreased stability of oils and oily foods. Acid value is one of the important indicators for evaluating the stability of edible oil and whether the processing technology is reasonable. In chemistry, the acid value (or neutralization value, acid value, acidity) represents the number of milligrams of potassium hydroxide (KOH) required to neutralize 1 gram of chemical substance. The acid value is a measure of the amount of free carboxylic acid groups in a compound (e.g., a fatty acid) or mixture. A typical measurement procedure is to dissolve a sample of known amount in an organic solvent, titrate with a potassium hydroxide solution of known concentration, and use a phenolphthalein solution as a color indicator. The acid value can be used as an indicator of the degree of deterioration of the oil. The unit of acid value: (KOH) / (mg / g). Influencing factor The size of the acid value of the oil is related to the raw materials for preparing the oil, the process of preparing and processing the oil, the storage and transportation method of the oil, and the storage and transportation conditions. For example, mature oil seeds are less mature or are germinated and mildewed. The triglyceride is decomposed by the action of heat or lipolytic enzyme during the oil production process to produce free fatty acids, thereby increasing the acid value in the oil. During storage, the oil is decomposed into free fatty acids in oil due to factors such as moisture, temperature, light, and lipase, which increases the acid value and reduces the storage stability. Measurement methods GB/T 5530-2005 specifies the method for determining the acidity of animal and vegetable oils, including hot ethanol determination method, cold solvent determination method and potentiometric titration method. The hot ethanol method is the reference standard method, and the cold solvent method is only applicable to the light color oil. Potentiometric titration is to determine the end point of the titration using a pH meter, and then calculate the acid value of the oil according to the amount of potassium hydroxide required for the titration. Folding titration 1. Experimental reagents and equipment Reagent (1) Neutral alcohol-ether mixture: 95% ethanol (CP) and diethyl ether (CP) are mixed in an equal volume of 1:1, and then a few drops of phenolphthalein indicator are added to the mixture, and 0.1 mol·L-1 KOH is used. The solution was neutralized to red. (2) 1% phenolphthalein indicator: formulated with 70% to 90% ethanol. (3) 0.1 mol·L-1 KOH standard solution. (4) oil (lard, soybean oil, etc.) experiment equipment Electronic balance; 25 mL basic burette; 100 mL Erlenmeyer flask (conical flask). 2, experimental operation Accurately weigh 1g ~ 5g of fat in the Erlenmeyer flask, and take a conical flask without adding grease as a blank. Add 50mL of neutral alcohol-ether mixed solution to the two bottles, shake and dissolve (solid fat needs water bath to dissolve and then add After mixing in a human solution or in a 40 ° C water bath, add 1 drop or 2 drops of the phenolphthalein indicator, and set it with a 0.1 mol·L -1 KOH standard (the KOH concentration depends on the degree of fatty acid loss) to a light red color, and the color is not faded for 1 min. At the end point, the amount of 0.1 mol·L-1 KOH was recorded. 3. Calculation Acid value = c(V2--V1)X56.1∕m Where: c is the standard KOH solution concentration, mol·L-1; V2 is the number of milliliters of KOH solution consumed by the sample, and V1 is the number of milliliters of KOH solution used in the blank, ml. m is the sample mass, g. 56.1 is the number of milligrams of KOH contained in 1 mol·L-1 KOH 1 ml, mg. 4, matters needing attention (1) Two parallel samples were taken for each sample, and the results were calculated as arithmetic mean. When the acid value is below 2.0mgKOH·g-1, the relative deviation of the two equal samples shall not exceed 8%. For other values, the relative deviation of the two parallel samples shall not exceed 5%, otherwise it will be redone. (2) When determining the acid value of castor oil, only neutral ethanol is used without a mixed solvent. In order to solve the problems of the traditional alkali titration method, many researchers have improved the method. Test paper method In order to improve the detection efficiency of acid value of edible oil and realize on-site inspection, Guilin Zhonghui Biotechnology Co., Ltd. developed a test paper for rapid determination of the acid value of edible oil. The principle of this method is to use the free oil produced by rancidity of edible oil. The fatty acid reacts with the agent on the test paper, and then the color of the test paper is compared with a standard colorimetric block to determine the degree of rancidity of the edible oil sample. The research and development personnel developed a test paper colorimetric rapid method to determine the acid value of edible oil. The method uses the free fatty acid produced by the rancidity of edible oil to react with the pH reagent in the test paper. The color change of the test paper reflects the acid value of the edible oil. The change, the degree of change is linear with the degree of rancidity of edible oil, and the effects of temperature, time, oil type and color on the measurement results are studied. The results show that the acid value of the acid test paper varies considerably from 0 to 5.0. It is easy to distinguish by macroscopic colorimetry, and the test paper is not sensitive to temperature, reaction time, type and color of oil sample, but the test paper method has the disadvantages of poor stability and slight error, and generally only applies to qualitative or semi- Quantitative testing. Colorimetric method The method comprises mixing an organic solvent (isooctane), a surfactant [sodium bis(2-ethylhexyl) sulfosuccinate] and a small amount of water in a certain ratio to form an optically transparent stable reverse micelle system, and the phenol Red is dissolved in the aqueous phase of the reverse micelle pH=9. When pKl is equal to 7.8, phenol red is red in alkaline medium, and its aqueous solution has maximum absorption at 558nm. The free fatty acid content is calculated by the standard curve. The method has high sensitivity and fast measurement speed, and is suitable for the determination of low fatty acid content. Sample and need to quickly determine a large number of samples. Chromatography The method first uses a solvent such as ethanol to analyze free fatty acids in oils and fats. Since fatty acids generally have high polarity, low volatility and poor thermal stability, they are generally converted into corresponding derivative fatty acid methyl esters by KOH/methanol. The polarity was lowered and its thermal stability was increased, and then analyzed by an Agilent 4890D gas chromatograph using an FID detector with a recovery of 89% to 109%. The method has a small amount of reagents and is suitable for large-volume sample determination, but gas chromatography requires a standard as a control, and the method is more suitable for determining the content of individual fatty acids and fatty acid components in the sample. Near infrared spectroscopy ChenMan et al. used 0.15% (w/w) esterase to digest natural palm oil in a constant temperature water bath to prepare palm oil with different free fatty acid concentration gradients. Using near-infrared spectroscopy, a calibration model was created by multiple linear regression. Correlation coefficient r=0.99; as long as the absorbance values ​​at l, 882nm, 2, 010nln and 2,040nnl are known in the model, the free fatty acid content in palm oil can be obtained; after 8 samples are verified, the predicted sample is correlated. The coefficient r=0.99, the method is faster, the total analysis time is 5min, and the environment is mild. AhmedA1-Alawi et al. developed a Fourier transform infrared spectroscopy (F1 dish) to quickly and accurately determine the content of free fatty acids in edible oil. The method has good repeatability, and its standard deviation sD is 0.029%, slightly better than 0.038% of the AOCS method has a good fit with the traditional titration method, IR/titration=0.99996 FFA (standardaddition)+0.047, SD=0.030, r=0.9994. Although the near-infrared measurement method has many advantages, the instrument is expensive, and it is necessary to use a chemometric method to establish a mathematical model between the spectral characteristics of the standard sample and the content of the measured component. At present, the method is applicable to the determination of the acid value of the edible oil. less. Potentiometric titration Potentiometric titration is a classical analytical method with the advantages of simple equipment, simple operation and high precision. Since the 1960s, the rapid development of ion-selective electrodes has provided a good number of indicator electrodes for potentiometric titration, improving sensitivity and selectivity. The accuracy of potentiometric titration for the determination of acidity or acid value is higher than that of the general titration method. It is suitable for the titration analysis of colored solutions, turbid solutions or the absence of suitable indicators. The measuring principle is that a indicating electrode (glass electrode) and a reference electrode (cabbage electrode) or a composite electrode are inserted into the oil sample solution to form a primary battery, and the magnitude of the electromotive force is related to the hydrogen ion concentration of the solution. When measuring the acid value, the mV value (or pH value) of the solution is continuously measured by a pH acidity meter during the determination of the oil sample solution by the standard alkali. With the addition of the titrant, the concentration of the free fatty acid occurs due to the neutralization reaction. Constantly decreasing, thus indicating the corresponding change of the electrode potential. When the end of the titration is near, the indication electrode potential jumps, and the measured mV (or pH value) produces a sudden change, then the measured mV value (or pH value) Titration consumes the volume of the lye and makes a titration curve to find the volume of the lye corresponding to the end point of the titration and calculate the acid value. ways to improve Although the above titration method solves the problem of accurate judgment of the end point of titration, it still requires a complicated titration procedure and a large number of chemical reagents. Therefore, the researchers have a solution of triethanolamine in a concentration of 0.20 M in water and isopropanol (1:1). In the development of a pH meter, the acid value of the edible oil is measured without titration, and the free fatty acid in the edible oil is quickly extracted into the solvent by the emulsification property of the solvent, and the pH value of the emulsified solvent is measured, and then converted into an acid value. The method can be used for quality evaluation in edible oil production and circulation trade. Its main advantages are: compared with standard technology, it takes less time and saves manpower; compared with chromatography and infrared spectroscopy, the instrument is simple and easy to use; Easy to automate. A technique for determining the acid value of edible oils by indirect titration in water and ethanol media has also been proposed. The accuracy and accuracy of this technique is suitable for the monitoring of edible oil quality and does not require prior neutralization of the acid. L. Lykken et al. studied the determination of free acidity and acidity in edible oils and lubricating oils by potentiometric titration. Potentiometric titration is generally considered to be one of the alternatives to traditional alkali titration, but it requires repeated measurements and more stringent processing, as well as cumbersome titration and calibration procedures that require relatively large amounts of oil. Double 4-layer Shelf,Tv Stand,Wine Cabinet Jiangmen Lihua Import & Export Trading Co., Ltd. Jiangmen Lihua Furniture., Ltd. , https://www.jmlihua.com
Determination of acid value of oil and its use value